Both of those (along with a majority of releases most months) are flavor of the month.
R6 Vegas 2 sold under 140k it's second month down from 752k, and in AoT's case, it didn't even chart in the top 20 the next month. The 360's social aspect to gaming means that the same people buy these games every month and then throw them away (not literally) when the new hotness comes to town. Not the case with multi-player games on the PS3. Killzone charted and did the same numbers it's first two months, and to this day still has 80k people/week playing down from the high of 500k at release. That's good for a game that is 8 months old, and been through Resident Evil 5, and now Uncharted. We'll see what the future holds with it and MW2, but I'd be hard pressed to believe that either AoT or RSV were being played 8 months after launch. I'm pretty sure Halo 3 stopped that from happening. Also, inFAMOUS went up sales in it's second month which is rather unprecedented for the HD twins, and LBP managed to hold onto a large chunk of sales for it's second month even with the launch of Resistance 2 and W@W.
Borderlands will also suffer the same fate. Everyone is playing MW2 so that's where all the money will go this month. Easily.
Not that it's anything wrong with a 'Flavor of the month' mentality. As long as it sells good, who cares, but we're just talking about why 360 games sell 'more' in opening months, and it's all down to the social aspect. These flavor of the month games never have staying power, and in a few months, Borderlands will be a figment of many people's imagination.