Not trying to start a pissing match, here, just genuinely curious...
Did your body weight change during this time?
The fact that your bench went up absent any dedicated work on it is still a plus, but I question whether the increase was really all that surprising given that your OHP went up by 66% of its starting point. Hence my question about the body weight. I wonder if your results are:
1) "working hard on OHP increased my bench press more than I would have achieved by actually benching"
2) "I got significantly bigger and stronger overall over an 18-month period - without bench pressing - and then I could bench press more weight at the end of that time frame"