Trust me, this is normal. Especially with Deadlift and Bench.
For example, I deadlifted 465x5 last week, this week, I couldn't even get 475 off the ground.
Somedays are just off, and here are a couple of things to take note of that affect me at least:
1) Did I start any new lifts previous to the day I was weak? For this week, I did shoulders for the first time in like 8 months, my lats were killing me, hence, crappy dead day
2) Sleep is huge for me, if I'm tired, I am weak
3) If I eat light before my workout, it suffers. I force food down
4) The approach to the PR is huge for me also. I can't expect to do 5 sets of 550 on squat, then just toss on 625 and expect it to go up according to a 1rm calculator, I have to ease myself into it with doing a heavy triple, double, singles, up to my PR. It gets your body used to the heavier weight and mentally ready.