If a mountpoint wasn't an empty directory before mounting another filesystem on it, the contents of that mountpoint will be shadowed by the contents of the other filesystem. So, if you mount other filesystems onto the one in question, this could cause your discrepancy, since the files are still there, just not present in the virtual filesystem tree. It's a common practice to set your mointpoints to immutable after creating them for this reason.
Somewhat more uncommonly, ext[234] allow the administrator to reserve a number of blocks for use by, in most cases, root — maybe you tune2fs'd your filesystem with some crazy value?