The auction with the 8 second lead time was not during a busy time of day, or on a busy day of the week. I would think your bid SHOULD have been placed on time, however there are spikes of traffic into eBay.
A 'Not In Time' means AS placed your bid, but eBay was unable to accept and process it in those 8 seconds before the auction ended. For the second auction, AS placed your bid a full 30 seconds before end, so had much more time to process and record your bid before the auction ended.
Note that normally 8 seconds is plenty of lead time to place a bid. In fact, that's the amount I use (except on Saturday and Sunday evenings).