Because a 20 point difference suggests that the best estimate of overall cognition is the VCI. That level of performance also matches up better with his matrix reasoning score. I would suspect that a reasonable description of your DS's ability is that abstract reasoning, both verbal and nonverbal, are up in the neighborhood of the VCI, while concrete problem-solving (the other subtests of the PRI, and the PSI subtests) is significantly weaker (relatively!), in the region of the PSI, while rote memory is in the average range. So basically, the more abstract and conceptual it is, the better he does, which is the reverse of most NT learners.
I'm assuming the examiner used the standard WMI subtests. If so, I wouldn't be surprised if the WMI would have been much higher if Arithmetic had been used instead of Letter-Number Sequencing.
I'll note that this kind of interpretation is unlikely to be a part of most GT screening, largely because of the politics of allowing nuance in the admissions process.
