If you know how harddrives work, you'll know why it does this. Eventually after your drives have been used for a while, Steam has to allocate the space for the files you're about to install / download. When you delete something from a harddrive, it doesn't remove it. It only marks the thing you removed as overwriteable. So when you have to download and install new things, you computer goes over the harddrive and finds enough of these spots to download your file. That's what allocating is essentially.