WordPress System Tables dynamically changing?

I do mysql backups frequently, and I ran one last week, and just ran one again this morning. Despite that new blog posts and comments had taken place since my last backup, today’s mysql backup was actually smaller (this is a standard mysql with no compression). I noticed during my first backup, when looking over the resulting file that WordPress contains a lot of table data that appears to be system-oriented or other things unrelated to my categories, comments, posts, users, etc. Does WordPress update the data in these tables automatically, whenever you login to the dashboard, for instance, that might explain why my backup was 10% smaller today than last week, despite more information on my end?