Running up against the same problem we had last year, although with an entirely different person.
They test DS2 in a small quiet room, no distractions at all. He tests well - has a scary memory with stellar recall, however, doesn't necessarily understand what he is memorising or repeating IYSWIM.
The SALT then says "oh speech and language skills and cognitive skills are just fine for his age." But they're not. The last SALT gave him the same flash card test for pronoun usage repeatedly during the year - he memorised the cards and the responses she gave him when he got them wrong, so eventually he could recite it 100%. But he was still using the wrong pronouns both in class and at home (verified by both myself and the class teacher). When we pointed this out to SALT, all we got was a shrug of shoulders and "well, he did it wonderfully when I tested him recently." Well, yes, because he memorised the corrected answers you gave him. 
So here we sit with a SALT report basically saying his cognitive skills are good. The teacher agrees that he does not display this level of skill regularly in class, mainly because he does not have the level of focus needed (due to impulsive behaviour and distractions) to access the skills.
I suppose technically the skills are there, however, the SALT does not qualify in the report that the tests are basically in a completely isolated environment - so it's very misleading IMO. Do they really not see the issues here?
I'm just frustrated as I'm in the process of putting together his DLA renewal and I just KNOW this will mean extra work to clarify this issue. Grrrrrrr....