Why do the free registry cleaners differ in the number of errors they find?

Registry Mechanic 365 errors
Registry Cleaner 170 errors
I could go on, one even told me I had 1007 errors.

Because there’s a trade-off between registry cleaners being safer and missing some things, or being more thorough and overzealous.
In my opinion, CCleaner is mild and safe, the registry cleaner in Advanced System Optimizer is more thorough but still pretty safe, and one called RegSeeker is more on the overzealous side.
Here’s a site reviewing registry cleaners:

http://lists.thedatalist.com/pages/Registry_Cleaners.htm

http://registry-repair-software-review.toptenreviews.com/

Which one came up with 1007 errors?