Many of these 40 times are fake, or at least poorly timed. The thing is, technology has improved. We no longer have to time forties by saying one-one-thousand, two-one-thousand, etc.
My players always want their forty times during summer workouts. Sometimes the grass has been cut recently; sometimes not. Sometimes someone bumps the cone and makes the 40 yard dash a yard or two shorter or longer. Some random coach with a stopwatch on his cell phone will begrudgingly time him without trying too hard. But guess what? When someone asks their forty time in a couple days, they give them that number...
...or they inflate (deflate?) it by .1 or .05 to make themselves sound faster.
We had a kid transfer in from Mississippi last year who told everyone he could run a 4.2 forty.
So you're right - there are all kinds of ways to measure the forty, and not one standard that we can all take to the bank.
I think everyone's standard is the NFL Combine. My standards lately have been Darrius Heyward-Bey and Pat White.
I hear about recruits saying they ran a 4.34 and I'm like, "Heyward-Bey was the fastest guy at the NFL Combine this year, and he ran a 4.3. That guy is in his early twenties and has been working for the NFL Combine for the last half of a decade, working with people who make their living on getting him as fast as possible. And you're telling me some random high school kid who did a few squats and chased some rabbits is just as fast?"
At the same time, people say, "OMG Tate Forcier runs a 4.55 because his Rivals profile says so and he's so fast he's going to change the game of football and win the Heisman OMG." Pat White has been tearing up college defenses for four years and training for the Combine, and the dude just ran a 4.55 that earned him millions of dollars. Forcier - who admitted to rarely if ever lifting weights - is not the same speed as Pat White.