you using abs ?
it will shrink so sizes are usually wrong enough to make a difference.
One of the many reasons I gave up abs.

Failing that - are you using mm or inches for sizing your bolts ?
Even so called 'equivalences' rarely match up.

One of those videos you linked to had a chap print all his bits, then go buy and cut the conduit, only to discover it was too large.
Me I'd have printed one part, taken it to the hardware store and checked it agaisnt the existing conduit before either printing the rest of the parts or buying the conduit or cutting it.

When I design parts I usually print cut down test pieces to make sure everything will fit when i print the full parts.

I think over time you just develop better working habits based on previous cock-ups :-)