Once one starts doing a little research into this, it seems likely that the CFL times are slower because they are more realistic.
Take a gander at this article:
The NFL treats 40-yard dash times as sacred
Mark Zeigler, STAFF WRITER // April 20, 2005
Copyright 2005 The San Diego Union-Tribune
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There is no official world record for 40 yards.
The shortest distance that the IAAF, track and field's international governing body, recognizes for world-record purposes is an indoor 50 meters, or about 54 yards. It is 5.56 seconds and it was set by Canadian sprinter Donovan Bailey in 1996. There is also a world record for 60 meters -- 6.39 seconds by American Maurice Greene in 1998.
But it is another Canadian, Ben Johnson, who is believed to have run 40 yards faster than any human in history. Johnson is best known for injecting copious amounts of steroids and winning the 100 meters at the 1988 Olympics in Seoul in 9.79 seconds, only to have his gold medal and world record stripped after failing a post-race drug test.
Timing officials have since broken down that famed race into 10-meter increments, and Johnson was so preposterously fast that he went through 50 meters in 5.52 seconds and 60 meters in 6.37 -- both under the current world records at those distances. He went through 40 yards that day in 4.38 seconds.
He was running in spikes . . . on a warm afternoon perfectly suited for sprinting . . . with a slight tailwind . . . with years of training from arguably track's top coach, Charlie Francis . . . with Carl Lewis and six others of the fastest men on the planet chasing him . . . with 69,000 people roaring at Seoul's Olympic Stadium . . . with hundreds of millions of people watching on TV . . . with the ultimate prize in sports, an Olympic gold medal, at stake.
And, as we learned later, with muscles built with the assistance of the anabolic steroid stanazolol.
Then again, maybe Ben Johnson isn't the fastest 40-yard man in the world.
Maybe half the NFL is faster.