I had the same problem when using the rep calculator, I think it's only accurate on low-rep calculations. My last 1RM bench test was based on 215x5, 1RM calculated was 250, I was working up to it, 235 went up no problem, jumped to 245, and only got a couple inches off my chest. I probably should have done 240 first, but I was expecting a 250 max that day, so I did bigger increments.
Try getting down to 5-6 RM and calculate your 1RM from there and see if it's any different.
Jumping from 10 reps to 1 rep is a big step, try using a program like Wendler's 5/3/1 to give some variety to the reps/set.