About tactics in1st and 2nd legs

I don't understand something about tactics in knockout stages.

During the last champions quarter-finals I've knocked out a team which was a bit weaker than mine: 49.2 vs 44.8.

I won 4-0 the first leg at home.
My 3-2-2-2-1 butterfly won his 4-1-3-2 from all points of view.

Return leg away, exactly the same formation with just 1 variation: normal mentality without counter instead of defensive+counter attack.

Well, after 80 minutes he was winning 3-0! I've scored one goal just at 85'!
And, please don't laugh, he has scored the second goal in 10 and the third goal in 9 because of two injuries!
I must suppose that in 11 the final result would have been 8-0.

This is the third time that it happens to me: I use the same tactic in the second leg (2 times at home, 1 tiem away) and the result was exactly the opposite.

So what? It is not advisable to use the same tactic for both matches?