How does this work?

Yesterday I checked in a patch for the MySql module which corrected the prototype for Mysql.mysql.create().

The reason I found that broken prototype, was that when fiddeling with the mysqls dbtype (see checkins from yesterday) the Sql module failed to compile since Sql.mysqls (which inherits Sql.mysql) overloads create(). The compilation failure was due to too many arguments passed to Mysql.mysql.create(). How can it be that that prototype has been broken for quite some time and Sql.mysql hasn't failed to compile? Can someone enlighten me a bit on how this works?

The feeling I got from the problem, is that it might be some weird bug, but it might also be that I fail to see the difference in the inheritance.