WHY DOES 3 REPS MAKE ME STRONGER?

this is a serious question ive been lifting weights for awhile now and i used to do like 9 sets for a specific bodypart like for example my chest workout was bechpress 5 sets of 6-7 reps and finish off with dumbbell flyes 3 sets of 8-9 reps and i would do that once a week and i used to get progressively stronger little by little but as of recently when i started lifting again like that i noticed my max bench went down dramatically after one workout so i tried just maxing out 1 time each week i did exactly this... 1 warmup rep of 160lbs, then 2 minutes later another warmup rep of 210lbs, then 2 minutes later i would put as much weight on the bar as i thought i could lift and THAT WAS IT! just 3 sets of 1 rep and the very next week i gained 5lbs on my benchpress each week after for month straight is this normal? should i be getting this much stronger this quick with such a small workout?? i also noticed this same thing with my deadlifts and curls too i do 3sets of 1 with the last set being my max out and i also addel 60lbs on my deadlift in 1 month!!?