MySQL rows in explain extended keeps increasing while the rows are being deleted continuosly
Is this the real query?
select count(*) from JobTable where userid=&quot;1&quot; and status=&quot;ready&quot;

If so, here's how it works when you have INDEX(userid, status) or INDEX(status, userid)

It drills down the index's BTree to the first entry with that pair of values.
Count 1.
Scan forward until the pair no longer matches (else back to step 2).

The only thing that can change is the details of the BTree.  INSERTs and DELETEs may lead to block splits and block merges.
Don't trust the numbers in EXPLAIN.
In this case, there is no benefit in using FORCE/USE INDEX; the index to use is too obvious.  (Or do you have a case where it used some other index?  If so, let's see SHOW CREATE TABLE.)
While we are at it, neither ordering in the INDEX is better.  That is, the cardinality of each of the two columns does not matter.
ANALYZE TABLE JobTable will refresh the statistics; it is fast, but imprecise, for InnoDB.