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While trying to combine to Gmail filters, I noticed something strange: filter from:person@example1.com alone returns 180 results, filter from:person@example2.com alone returns 69 results, but when combined—from:person@example1.com OR from:person@example2.com—, the filter returns only 120 results. Why?

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For me, using OR, it returns the same number of results as the search for person2. Internal algorithm issues. – Bibhas Jul 3 '12 at 19:05

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Did you validate the actual numbers? The count of results is an estimate, indicated by the word "about". In the version of Gmail I see today, for a search like your first one, I get:

1 to 20 of about 192

But when I page through the results clicking the > button, the counts change:

21 to 40 of many ... 41 to 60 of many ... 61 to 80 of about 172 ...

If I keep paging through, I arrive at the precise number:

221-238 of 238

It seems logical to me that combining another search with OR might give a lower number than both separately, since they're estimates.

Another explanation assuming result numbers are precise: the search hits are on message threads. It's possible that a thread can contain messages from:user1@example.com AND from:user2@example.com -- can you check that angle?

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The results were without “of about”, so not estimates. – Alex Oct 3 '12 at 20:27
Gmail evolves all the time - what do you get today? That is, who's to say they weren't estimates? Did you validate the actual numbers? – Fuhrmanator Oct 3 '12 at 20:28
Yes, I did. And I don’t know what I would get today, as I don’t remember the searches I made at that time. – Alex Oct 3 '12 at 20:36
The only precise numbers (without "about" or "of many") on the searches I get today are when there is less than a page (less than 20) of results. I'll make a theory that Gmail added "about" recently to the results message to resolve the inconsistency you found. – Fuhrmanator Oct 3 '12 at 20:41
I just checked it with two different strings and for the first one I get 2 results, for the second one I get 9 results, and for both of them (OR) I get 12 results. – Alex Oct 3 '12 at 20:47
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