How common is it to have to 'make' 5/16" bolts fit into the holes?

5/16" bolts fit (snugly) into some of the bolt holes in my parts (that I printed on my CR-10).

However for about half the holes the fit was tight enough that I needed to turn the bolts so they threaded their way into and through the holes.

And for a few holes I actually took a 5/16" drill and manually used it to make the holes bigger.

Is this common?

I have the 3D printer running in my living room, and my roommate has lots of windows open, so there is a fair amount of air flow (and changing amounts of humidity) during these long prints for the parts - could this explain why some holes are perfect, and some are a bit tight, and some are too tight?

I assume that if my printer wasn’t calibrated then all the holes would be wrong.

It can happen, temps will play a part in this. To be safe print a large 100mm+ test object. Knowing for sure is priceless, it can save many headaches and a ton of print time down the road.