NEW BOB INTEL: (Maybe it's not new intel, but I haven't seen anyone else mention this at all) I'm not sure if anyone else was aware of this, but Bungie records WILL keep track of multiple kills off the same Bob. I checked my kill records, and I've killed a total of five Bobs and been killed by one. I've killed three Bobs on Winter Contingency Normal, been killed by a Bob on Oni: Sword Base once on Legendary, killed one Bob on Nightfall on Legendary, and killed one Bob on Tip of the Spear on Legendary.
It is confirmed though, Bungie records will tally multiple kills off the same Bob on the same difficulty, as I have three Bob kills on Winter Contingency on Normal. I cross-referenced my different maps with different difficulties.
So make sure that your Bob kills aren't duplicates. Whatever your record shows, go through specific maps and difficulties to see which Bobs are repeat kills.
[Edited on 09.28.2010 6:41 AM PDT]