Why does Google Scholar provide a different number of references when sorting by relevance versus date?

Question

I've noticed that sorting by date, and relevance has different numbers of results. Why is this?

Edit: Here is a reproducible example:

Here is "foo" sorted by date (15,000 references)

https://scholar.google.com/scholar?scisbd=2&q=foo&hl=en&as_sdt=0,5

Here is "foo" sorted by relevance (300,000 references)

https://scholar.google.com/scholar?hl=en&as_sdt=0,5&q=foo

Answer 1

(I'll repeat an earlier answer of mine, which is relevant here, although the question is not a duplicate. Nevertheless, the question itself is relevant. Why are you interested in how many Google Scholar results a particular search yields in the first place?)

The "approximate" ("about") number of hits Google reports is completely worthless - in all Google searches. To see why, look at this number on both the first and the tenth page of Google hits:

First results page

Tenth results page

When I just did this, I got "approximately 15,200,000 hits" reported on the first results page... but only "approximately 96 hits" on the tenth page.

Given the opacity of Google's search algorithms, which likely have a lot of sampling built in, it's not overly surprising to me that you get surprising results even if you only sort your results in a different way.

In addition, your results will probably vary depending on your search engine bubble (another reason why this number is useless).