I'm a long-timer with e-rewards, and also have periodically received an offer that had expired even though I responded seconds after the offer hit my inbox.

I contacted e-rewards about this a while ago. The explanation I was given is that certain offers may go out in waves, not necessarily all at one time. So the offer that "just expired" may have been part of a late/final batch of mailings, so perhaps even by the time it was sent out the quota had been filled by earlier respondents.

That was their story, anyway.