Even executing a Lead turn does not mean that you can catch your opponent if both aircraft are of the same type and have the same loadout. To catch your opponent in a straight chase means that you overtook them with greater speed. What's really happened is that the Lead turn ends within firing distance, and that airspeed is still great enough to allow for a window of opportunity before the enemy's greater speed creates too large a distance of seperation.
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P.S. The antidote to lead turns is to turn into the bandit. A lot of the time you're below their nose when they're in the lead turn, and on occasion I've caught someone who initiated their lead turn too early for a perfect belly shot.