Today I have received vox populi badge, but I haven't used maximum 40 votes today nor yesterday nor in last 7 days and I think I have never done it. So why I received it?
We just deployed a change that affects the voting badges. This is why you see a number of these badges being awarded.
For more information about this change, you can see this answer.
Note that there is a bug with Vox Populi only that is awarding too many of them, beyond the expected more for retroactive awards, and a fix is being deployed now. The erroneous awards there will be cleaned up shortly.