Understanding the Result

That is an incorrect “interpretation” of the result received. Each proficiency level for the ASLPI has a large range. Understanding the range becomes more clear when you consider such a limited scale (0-5) being representative of all skills ranging from knowing no ASL to high level proficiency for such a rich and complex language.

An examinee can make incremental language improvements but when the ASLPI is taken, the level received could end up being the same given the range for each level. Examinees move up and down a proficiency level within the range until language improvements/incorporation are significant, complexity is increased, and increased accuracy in the language is demonstrated.

The ASLPI captures overall language skills at a given point in time and examinees often fall within the same proficiency level (up and down the range) for a period of time until increased accuracy, consistency and complexity in the language are achieved.
Understanding the difference between progressing in the ASL course sequence and moving up the proficiency level scale on the ASLPI is important.
ASL Courses... ASLPI...
...introduce specific features of the language and test on those specific features. Depending upon where in the course sequence you may be, many or some of the language features are not yet part of your language repertoire. They have not yet been introduced, they are not fully understood, or they are still in the emerging stage. ...is evaluating ALL of the features of the language.
...permit practice and re-do's of projects, assignments, videos, etc. and feedback is provided so students can fix errors as the course progresses -- all part of the learning process. The topics discussed and signed about become familiar and language skills begin to improve on those specific topics given the familiarity. ...is an impromptu conversational exchange which covers a range of topics to give the examinee every opportunity to demonstrate language skills with accuracy and increasing complexity. Topics, both familiar and unfamiliar, are also included to identify both the maximum skills (ceiling) and limits (floor) of an examinee's functional language ability. This evaluates stability of language skills and use of language features across a range of topics. Each evaluation covers a wide range of topics to observe the examinee's accuracy, consistency, complexity and flexibility in language structure, vocabulary and production. A range of topics and change in topic is also essential to evaluate flexibility of comprehension.
"I have been signing for a while and was expecting a higher proficiency level. Can you explain why my result is not as high as I thought it would be?"
American Sign Language (ASL) is a rule-governed language just as spoken languages are. ASL is a language completely separate and distinct from English. It contains all of the nuances, fundamental and complex features of language. It has its own rules for word order and grammar. Semantics play an essential role for choosing correct vocabulary for intended meaning. ASL has its own set of colloquialisms and cultural references. While every language has ways of signaling different functions, such as asking a question rather than making a statement, languages differ in how this is done. For example, English speakers ask a question by raising the pitch of their voice; ASL users ask a question by raising their eyebrows, widening their eyes, and tilting their bodies forward (grammar indicators).

The next aspect to consider is language proficiency needed for effective communication. The higher language proficiency someone has, the more able they will be to handle the range of language
Language registers include:

static (Pledge of Allegiance, Preamble to the Constitution, laws/policies);
formal (presentations, lectures);
consultative (teacher/student, doctor/patient, attorney/client);
casual (friends, coworkers);
intimate (sisters, partners, husbands/wives);
, as well as meet the varying communication needs of the persons to whom we are addressing. If we look more closely at the continuum of signing, ASL is on one end of the continuum. All along the continuum are communication modes which are varying combinations of English and ASL. Once someone achieves fluency in language (in this case -- ASL and English), it is possible to choose and use language features to meet the needs for the audience to which we are addressing. We will also have the ability to raise and lower our language proficiency to meet the needs of someone who has higher or lower language proficiency.

The ASLPI is not evaluating the "communication modes" along the continuum; the ASLPI is solely evaluating ASL (rule-governed language). Many signers have conversational ability; however, most have language errors unbeknownst to the signer. Their communication abilities may be sufficient for some situations, but lead to misunderstandings for others. The ASLPI is evaluating the accuracy, consistency, complexity and flexibility of functional language.