To be honest, I don't know the exact details on how mount works, but the files that you speak of - the ones that are present in a directory before you mount... they still exist, their parent directory just points somewhere new now, so there's no way to get to them. As soon as you unmount, their parent directory no longer points elsewhere, therefore, you can get to them again. Remember, directories are just special files, so mounting to a directory is kinda like modifying a file so that it does *something else* when you ask for its data.
anecdote: At one company for which I did some work, I had a RAID array that mounted to /job. If the array didn't mount, /job just pointed to the system disk, so I put some files on the system disk in /job that contained instructions on how to remount the array. /job was an nfs export, so if someone went looking for raid data via /job & the raid wasn't mounted, they would have instructions on how to get the RAID back. Once the RAID was remounted, the instructions would magically disappear.