Your fixed raid has two agility users
The other agility user already has the item
**LFR and Normal/Heroic have the same internal % chance to select that piece of loot to "drop"
Say a 2% chance to drop
IF it drops, 100% to receive
Your chance of getting it on any given week after the first guy received it = 2%
You have a 15% chance of receiving loot at all
2% "drop" rate, the chance that the rare item is the one chosen to put in your bags
0.15 x 0.02 = 0.3% chance of getting the item and it NEVER changes.
Both formats have a 2% "drop" chance on the item. Fixed raid group has a better "win" chance.
Obviously, there are assumptions here that can be wrong, with probably the most important one being the starred one. I've never seen any data to contradict these assumptions, however, so this is how I've assumed it works. Given my resto druid ran a T13 heroic 27 times to even see his first off-hand drop, I have no problem with the way any of my assumptions look. Seems to me, the difference between the two sides of this conversation are in the assumptions which neither of us knows absolutely.
"he doens't need healing, he doesn't need healing, he doesn't nee-WHAOSHIT!wtf was that man!". Please stop leaning on TDR. -Teng