Dealing with non-cononsistant lifts

I know sometimes we have good days, and bad days when if comes to training. Based on a 4 day week workout where you hit a set of body parts a certain day. How much do it hurt your gains, either muscle or strength if you don't hit the same rep or weight #'s from previous week? Example, Going heavier on a bench press than normal , but that hurts your dumbbell press numbers from week prior.