A 7/16 bolt would not “fit perfectly” in an M10 nut, it would really take some effort to cram that in. Not sure you could do it at all.
My guess is that a 3/8” bolt was misfiled in the 7/16” bin in the hardware store and that’s what you used. (You also say you used a 3/8 washer.)
I have noticed that a 3/8-16 bolt will fit almost perfectly in a M10-1.5 nut, a little loose, but easy to ignore if you’re not paying attention. So might also kinda fit M10-1.25. I have actually written warnings on my thread measuring display (search “thread checker” on Amazon) to remind myself not to do that.
My guess is that a 3/8” bolt was misfiled in the 7/16” bin in the hardware store and that’s what you used. (You also say you used a 3/8 washer.)
I have noticed that a 3/8-16 bolt will fit almost perfectly in a M10-1.5 nut, a little loose, but easy to ignore if you’re not paying attention. So might also kinda fit M10-1.25. I have actually written warnings on my thread measuring display (search “thread checker” on Amazon) to remind myself not to do that.