If you want your find command to exclude inodes belonging to other mount points, include -xdev on the find command line. The 78898 at the end of your output is probably more than the total number of files visible to find in the rootfs partition, because it includes files underneath other mount points. This much does not explain the discrepancy you asked about; it just means the actual discrepancy is probably somewhat larger.
The first thing to check is whether you have unlinked open files. The output of the following command, run as root, should show you those:
lsof +L1 One example of why you might have open unlinked files is processes which began before package upgrades which replaced them with newer files (a couple of sshd processes on one of my systems have open unlinked files). Another possibility is processes which deliberately unlink open files to conceal their contents from other processes.
If that doesn't fully account for the discrepancy, another possibility is that files which have not been deleted are still invisible to the find command because they are hidden underneath other mount points. There are various ways to investigate that possibility, but it probably makes sense to count open deleted files first.