I would guess because he got all averages or less. You can be a straight "Average" pilot and not wing because you need so many "Aboves" to meet the minimum GPA. It's not the MPTS system.
I know one or two folks I got winged with were sweating that in the HTs. As it turned out, they had no reason to, but everyone wanted to make sure they at least had the minimum number of Aboves to wing.
Just to clarify the point: you are saying that there is no minimum NSS with MPTS. That's the belief that I have (since I haven't found anything to say that having a low NSS is grounds for attrition) with the MPTS. For those non-IPs, let me expand on that...
With the old system the criteria for passing/failing was how well you did compared to your peers. Therefore the bottom slice of pilots could be attrited. With MPTS, the criteria for passing is not how you compare to others, but how you compare to the standardized requirements (aka "making MIF"). If one makes MIF by the end of block, but didn't have any greater-than-MIF scores and perhaps only made MIF on the very last flight, then his/her NSS would be pretty low for that block. However, he/she is still passing. Therefore by the time selection rolls around, the student has passed primary but has a very low NSS because of his/her performance compared to the peers. This next part is simply my theory...
There should be the same number of high scores (think 60s and 70s) that there are low scores (20s and low 30s). Why doesn't this happen? My guess is that the grades of those that attrite still count toward the squadron average. Therefore, because they were doing bad, they bring the whole squadron's score down per block. Since they fail out and don't make it to selection, their statistically crappy scores never get seen. Hence, it's rare but definitely possible to have a student struggle through but still pass and end up with a 20-something NSS.
*I should have read what FlyBoyd wrote before writing this. Oops.