I have a kid with a PG GAI that has had low PSI scores both times he's been tested with the WISC. In his case it is definitely more then him just being cautious, we see it play out every day with him so I don't have a reason to doubt the scores.

Where I live they won't give out composite scores, only 95% confidence intervals. The first time DS was tested (age 6.5) he was 9th%ile, the second time (age 8.75) he was 13th%tile. At first I thought - wow, that's better, at least he isn't in the single digits but when I look at the 95%CI ranges there is a lot of overlap. (74-90 for the 9th%ile and 76-93 for the 13th). It is also a 95% confidence interval not 100% so it is possible for them to get a very different score some other time. His VCI the second time around was WAY above the first round and the two ranges for the 95CI had no overlap for example.

Based on my experience I recommend using the scores as clues but if they don't match what you see in real life go with your gut. DS's second GAI score makes a lot more sense with what we experience with him. His low PSI also makes a lot of sense with our experience. Exactly how low it is and what the exact number is isn't the important bit - putting it together with our experiences so we were pointed in the right direction was.