in that case only things i can think of that your current HP values were different between each time, or the slight bit of randomness in damage kicked in (there's a tiny random mod on damage that's more noticeable when you do a lot of damage, like full bursting). If there's Crit buffs, then that matters too as there's a chance crit activates, not a guarantee
the raid boss is one thing, cause bosses have defense values that could possibly be different, but if one FB on andram did 5.7 mil and another did 6+, it's likely pride weapon did kick in more for one of those attempts.
your current HP is likely going to be different each time, and the more pride (and eventually vigor) weapons you have, the more noticeable this becomes. it's very rare for you to have say, 5254 HP on the same exact hime when you FB both times, let alone on all 5 of your party. all it needs is to be in a certain threshold so even 1 HP off might trigger more of a boost. only way to avoid that is to have exactly full HP each time you do it (like during burst time)
as for crit, i pretty much said that already. just another chance of fluctuating values (though i see it work a lot for nepthys and cybele)