Nors, I believe you just reiterated what I just said.
Anyways, hand held times are shown to be from anywhere from .15-.18 of difference in the actual times. In some cases, as much as .20 seconds.
Example would be DeAngelo Hall. All I heard during his last year at VA Tech is how he ran a 4.15 forty yard dash. The time was done on a track surface, with cleats, wind-aided, hand-held. He came to the combine and ran a little over 4.4 in the forty, which is nowhere near terrible, but nowhere close to the 4.15 forty yard dash.
But as shown above, other things like wind (yes, if the wind is at your back it can help), type of surface, shoes, etc., can be deceptive which trying to gauge a person's true speed.