Making compost free draining

I wasn't sure where to post this so I hope this is the right place. This is for potted plants when it specifies "free draining" compost. Why do they suggest you add sand to make it more free draining when sand holds so much water? even grit once wet stays that way for a long time so again is hardly free draining. If I want to make the compost I am using more 'open' I add perlite because that really is free draining, it doesn't hold water for very long. Has anyone else had any thoughts on this?

Val