Many different 40 times will be reported over the next few days, with discrepancies as wide as a full tenth of a second like those for Spiller, so it's important to understand where they come from.
The only listed times available during the event come from NFL Network and NFL.com, neither of which discloses how they arrive at the timed speeds.
As Rob Rang noted in his blog this weekend, beware of any 40 time labeled as "official" from the combine. Those who participate in the 40 typically run twice, and on each run they are timed by two hand-held stopwatches and one electronic timer, which is actually initiated by hand on the player's first movement.
Combine data put together for NFL teams by the National Football Scouting service, the private entity not owned by the league but which is actually in charge of running the event, includes all six of those times for each player, but no single official time. Teams and scouts don't receive that information until at least two weeks after the event.
Team scouts and coaches have various approaches for getting the 40 time they use from those six timings. Some use averages. Some throw out the slowest and fastest and then average the rest. Some ignore the whole thing and use a time taken by their own scout.
In deference to the players, NFLDraftScout.com uses the best verifiable --- or listed -- time from the combine unless it is conspicuously skewed from the other times, which happens when a hand timer has an itchy trigger finger on the stopwatch.
However, the times are usually well-grouped so a general idea of the player's speed is easily attained. As one longtime NFL scout once told me, "all I want to find out is if a guy is slow, fast, or damn fast."