Well, I can give you one example. Linux, EXT filesystem (reproducible on EXT3 and EXT4, didn’t test on earlier versions but shouldn’t be different), a file that is hard linked elsewhere. Some changes in the file happen elsewhere on the system (i.e. not in the directory we’re watching). Inotify has no way to spot the fact that the file we’re watching has changed and report it. Directory rescan, however, immediately sees that the file has changed.
I’m not really too much versed in how inotify works, but logical thinking suggests that since no access to the file in our watched directory was ever performed to begin with, there’s absolutely no way for inotify to get aware of the changes.