Why Do The Number of Results for a Site:Domain Search Fluctuate Wildly?

Google’s John Mueller today clarified that the number of results shown for a site: search (e.g. site:detailed.com) are “extremely rough approximations”.

A question appeared on Reddit asking why the number of results shown fluctuates so dramatically on a regular basis.

Here’s the exact quote,

Site:-query result counts are optimised for speed, not accuracy.

I wouldn’t use them for any diagnostics purposes. Especially when you get into the higher counts, those numbers are extremely rough approximations. Don’t get tricked into trying to use them for optimizations or diagnostics purposes.