Tell me I am not alone!!??

We just got or son's Stanford Achievement Scores and he did not do so hot :-(! Is this normal with 2e kids- huge difference with performance scores compared to ability?!
Our boy (2nd grader) is currently on an IEP for speech, Title I program for reading and we had him tested last year for learning disabilities (his reading and writing are coming along very slowly so wanted him checked out). The psychologist suggested getting him tested for Auditory Processing Disorder but we haven't yet (scheduled for next week- after we got the scores I felt we should go and do it). There was also concern about dyslexia (I personally do not feel this is an issue). He is way behind on reading (could this have impacted his scores, even the non-reading subcategories?)

What are some other reasons that there is such a huge gap. His ability and IQ has been tested more than once (through the school and with the psychologist when doing LD screening) so I do not think it is an accidental thing- his ability scores did improve but different test so I assume it is normal for slightly different results. His achievement seems to have declined between the last year!!!

I am at a loss