How come HN's search shows different counts based on sorting criteria?

I was playing around with the algolia search tool recently.

I noticed that if you ask for all items ordered by Popularity you get 38m results: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&query=&sort=byPopularity&type=all

But if you sort it by Date instead you get only 32.5m results: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&query=&sort=byDate&type=all

Can anyone help me understand this difference or point me to where it's been answered?