A student accidentally ordered a big slide (G21/30) Big Dots for her G19, when I looked at the packaging I told her it was not marked for what she needed.

However I slid the rear sites on and the front and seem to install just as fine.

Anyone know what the difference would be? Not to be liable, I changed it back to the factory plastic ones and told her to reorder the correct model, luckily I bought them since I needed a pair for my 21SF.

Afaik the height or recoil impact would change.

When I got my XS I was nervous it wouldn't work with my G20 when it wasn't listed on the package.  A lot of rounds amd a year and a half later I have had no issues.

 

Almost certainly heights. Big dots are such a coarse sight it probably doesn't matter for 90% of users.
Then of course there's manufacturing tolerances so it is entirely possible the gun regulates properly with that height and not the "proper" height for model.