Unusual behavior of ln command

I’ve noticed strange behavior with ln. I want to maintain one set of filters for thunderbird (betterbird), so I tried creating a hard link (symlinks aren’t recognized) from one account to the other, ie. ln acct1/msgFilterRules.dat acct2. It doesn’t complain, and a file is created, but I realized it actually, silently, created a copy not a link (they point to different inodes). OTOH, if I do, eg. ln acct1/msgFilterRules.dat ~/tmp and inspect the inode of that file, it is the same as the original ...