Its usually the delay because you can cancel bets at any time, the delay is only on placing/modifying them.
So even if just you and I were trading against each other from home using the same pics I could pull my orders before you took them, even if you clicked 1 or 2 secs ahead.
And of course, a lot of people do stack money all over esp on the bigger matches then pull at the last sec if it's not favorable to have it matched anymore