If you break it down into C, it kinda makes a bit more sense:
FOR(int countvariable = 1; countvariable <= 10; countvariable ++)
{
}
So countvariable is set to 1, and as long as it's <= 10, it'll increse countvariable and do whatever is inside the {}.
So, when countvariable is 10, it still increases it, because the ++ part happens each time it checks the for..next variable. When countvariable > 10, the loop is skipped. Countvariable has to equal 11 (or >10) to skip the loop.
It's typically a good practice to only use your for..next variable within the loop, once the loop is done, disregard it.
I got a fever, and the only prescription, is more memes.