I'm a strength and conditioning coach for athletes at a high school. However, I worked with pro, Olympic and collegiate ones, and I wanted to share some things. I know a lot of you will still gush over combine heroes, but take this for what it's worth.
Bench Press Test: After about15 reps or less than 70% of a max, the lift becomes a strength ENDURANCE test, not a test of absolute (brute) strength. For an example, my training partner can bench 455 lbs, but could only bench 225 about 24 times. My best bench press is 355 (raw) lbs and I could rep out 23-24 on any given day. Who was stronger? The truth is, I have better lb for lb endurance, but my partner is much stronger. This test really ticks me off because it's actually the most USELESS in telling anything about an athlete!
Secondly, the best test of strength isn't a bench press despite what people think. In just about all sports movments the power is coming from the hips. A better test should be a max set of 1-5 reps on the squat and/or deadlift.
The 40 yard dash: As discussed before this is MOSTLY technique. The starting technique is critical. I have seen people improve their 40 time TEN FOLD by just improving technique.
You know what else? Football players VERY RARELY go over 40 yards. It's like testing out a 100M sprinters 400M time. It's useless.
The Shuttle: Again, this is all technique too.
Vertical Jump: This is a lot of technique too but out of all the tests the jumps might be the most telling. Generally, the higher the jump, the more explosive one is. In fact, one of the exercises I and many other strength coaches uses to get a player more explosive is improving different kind of jumping variations such as box jumps. Still, it is technique and nowhere in football does a player do a straight up vertical jump or broad jump.
Like I have previously mentioned many players now a days are practicing this stuff in HIGH SCHOOL knowing that s trong combine outing could be the difference between millions of dollars. A really productive college player who produces well at the combine can sky rocket their draft status and sometimes it works out... Other times the team who drafted them gets duped... Beware of the combine #s, they're not a good test of predictability.