There are a lot of little and not-so-little things that mess with track-bred guys' ability to run well in a PT test. For guys in Basic it's a no-brainer - you're tired as hell and there's no way you could put in the kind of workouts you'd need in order to sharpen up even if you had the time and weren't having your mind and your ass beat down every day. I was at OBC, which by comparison is a joke and in my case was a six-week party punctuated by twice- or thrice-weekly PT that was also a joke of equal magnitude. I wasn't downtown with the party-boys every night and I was putting in maybe 60 miles a week with the occasional workout, so in theory I could have run OK in the test, but training in San Antonio in July and August is not a barrel of fun.
The sit-ups and push-ups may not "kill" you but they aren't an idea way to warm up for a race, either. Plus it sucks wearing the cotton T-shirt and shorts everyone has to suit up in, especially when it's 88 degrees and humid as hell even at 4:30 in the morning. These sound like minor gripes and they probably are, but the shit adds up.
When I did the test they started about 75 people on one corner of the half-mile track and another 75 or so at the opposite corner, so the faster guys in each group were catching packs of slower ones within two minutes or so. The size of the track cut down on the lapping factor, but it was still a predictable clusterf*** of the highest order.
When I was at camp there was a CPT stationed there who had run at Memphis State (now the U. of Memphis, I think) in the mid- to late 1980's and reported having run a 47-point 400 and 24:30-ish in cross as a collegian as well as an 8:57 duece in the PT test (he may have done this in ROTC before he was commissioned, I'm not sure). It was obvious from talking to him that he wasn't blowing smoke.
Someone else will have to confirm this, but I believe the cadet standards for getting max scores (100) in each event at West Point are, or were, 100 PU, 100 SU and 10:30 in the run.