Half of your Hot Lists show your activity on the site. Every time you look
at or contact another member, it shows up on your Hot Lists and
the numbers go up accordingly.
The other half of your Hot Lists tell you about what other members are doing with regards to you -- looking at your profile, emailing you, etc. Every time people look at your profile or contact you, their profiles show up on your Hot Lists, and the numbers go up accordingly. Keep in mind, however, that some members choose to hide their profile from showing up in your Hot Lists.
Also, in order to keep your Hot Lists up to date, we periodically clean out the members that have been in your "viewed" Hot List for a while. This also changes the counter.