why in explain in mysql row count is very high
Is the table InnoDB? For that the index statistics are gathered by "index dives" and are by definition imprecise.

EXPLAIN is not actually executing the query, but uses the estimates to plan it so it shows those imprecise numbers.

The important thing is that if it keeps numbers in the same order as reality (which, being a double in your case, it is), it works well enough for most cases.

Keeping exact numbers is too cumbersome and not even really possible in transactional environment.