It's not reserved as in it can't ever be used except for the recycle bin - if you need to store enough stuff on the disk such that the total free space is less than what the recycle bin has reserved, it'll do it anyway. That size is basically the maximum size the recycle bin can be on that drive, that's all. Of course, you still have to empty it manually if the disk becomes full and there's a few GB of data in the bin, it won't empty it for you if you attempt to put more stuff on the disk than there's space for with the bin in the way