Is your opponent active?
If he is active, then it's expected that the opponent may change tactics between the two legs. Even if the formation stays the same, the orders might change. Typically in FA matches (in higher leagues) where managers are more active, this is the case! I saw it plenty of times recently while my association was in Gold 1, when my team ran off 22 consecutive victories. First leg we would win like 4-1, 5-0 of so frequently, then 2nd leg it would be like 2-1. One time we played the same association two weekends in a row, against the same opponent; scores in order were 7-0, 2-1, 3-2 (trailed 0-2), 4-1. The opponent changes something every time. Used same formation for the first three meetings, then changed formation for the 4th. He was only level 4, so it was easier to figure out what he changed tactically than say if a higher level manager with more experience was managing. But this game engine is largely tactical I believe, and the person managing against you, they will change stuff if active, and even subtle changes could make a big difference.
The definition of insanity is, "to always do the same thing and expect a different result."