It is reporting 18 players, not 20. The number is the client's slot number, not a total count.
h-reservedslots has a bug in it:
Code:
new currentClientCount = GetClientCount(true)+1; // +1 for the freshly connected player which is not counted yet
The documentation for GetClientCount/OnClientPostAdminCheck says not to do this (see clients.inc). It's counting an extra player where there is none.