Several years ago, I was building a gun, and we had had a very unusually nice period of weather with low (30-some odd percent) humidity. I think it was May. I had just put the barrel tenons on and got them pinned to the stock, which was maple with curl the whole length of the fore end. I left the stock (unassembled) out in the shop when done. The next day, our normal humidity returned (like 65%) and I couldn't get the barrel pins to line up again! The fore end had stretched a total of about a sixteenth of an inch in one day! So, I learned to slot the tenons....
Now, if the grain was straight and plain in the fore end, it wouldn't have done that, but with all the curl, you get lots and lots of cross grain stacked up, and that expands and contracts like crazy