You may have needed it because the SlackBuild you were working off of had a premade slack-required call for 'sdl' lowercase. I ran into this as well, since I think 'sdl' lowercase is a very common way of installing sdl on slackware systems. SDL uppercase is how the package itself is named when you download the source.
Anyway, I bet if you...
you will find both.
I'm not an expert on slapt-get dependencies but my worry is that when one is removed either by removepkg or by slapt-get the other one will remain in name only in /var/log/packages/. This would then cause slapt-get to think it is there when it is not. Maybe someone more experienced can verify.