That's how --force-depends works - it still reports the errors it encounters, but completes installation anyway. When you run it the second time, the package is installed already according to opkg's housekeeping data, so there's nothing to do for opkg, and thus the dependencies are not checked again.
But of course, you still have a dependency mismatch in the installation, which might cause trouble when actually use the modules.
On the other hand, as long as the module in question is not using any bleeding edge kernel features, chances are good a third tier version mismatch does not matter.
Kernel dependency checking is so strict because if something does go wrong in the kernel, this might render the system unusable/unbootable up to the point that you need to do a low level debricking over ethernet. That's something you can risk (knowingly by specifying --force-depends) while experimenting, but don't want to happen otherwise. So while the strict checking is sometimes annonying, it makes a lot of sense.