There are differences between inch & metric, but it's not things like screw threads. Partly that's why the term "inch pattern" is used so much as its the design changes between inch pattern & metric pattern guns.Why do they specify inch or metric?
The way it worked was that there were 2 basic versions of the original Belgian FN-made FAL rifle, (we're ignoring the oddballs like the Izzy & Indian versions which were their own little world here).
Fabrique National gave the design to Britain in thanks for the help they received during WW2 & the British reworked several design features to be the way they thought was best. This was actually done in Canada for the British.
Things like the HTS, the pistol grip, the butt attachment, the lack of an automatic bolt hold open, the bolt carrier, receiver & bolt being modified to be more sand resistant, as an example. Things like screw threads were re-labeled to Imperial specs but the thread wasn't changed, just the way it was described.
Then the Austrians changed the metric pattern to a lower sight profile & the die was cast.
If you're curious google "User preference modifications to FAL rifles" & you'll find hundreds of pages of info.
Metric magazines will sometimes fit some inch magazine wells, but usually not properly. It can be inserted & will lock, but its position is off & the fit is sloppy, sometimes so sloppy the magazine will either fail to feed, or fall out under recoil.