On the question of picture concepts tracking with other subtests in general: my experience is that, in some children, it doesn't track with the other perceptual reasoning tasks, but instead tracks with the verbal comprehension tasks, as it is more accessible to verbal mediation than the other perceptual subtests. I also find that it is not as predictive of adolescent/adult performance on the older version of the Wechsler as the other perceptual subtests are, perhaps because of that and other qualities. It is a more concrete task than the others, which can be beneficial for students who are more concrete and conventional in their thinking (or understand that kind of thinking well enough to play the game).
Similarities, on the other hand, is a particularly abstract verbal task. Even though both subtests are supposed to be similar in terms of concept formation, starting from two verbal concepts is quite different from starting from two or three images of concrete, (supposedly) familiar objects. Picture concepts is also more vulnerable to exposure, experience, and cultural differences than the other two PRI subtests, because of it's concrete content.
And...one of mine couldn't figure out what letter to put in to complete the word fi_m, l or r, because neither one made sense with the picture of a camera.
