That is not over-provisioning, it's just that 'df' doesn't have the concept of pooled storage. With pools it's possible for different file systems to share their "available" space. BTRFS also has its own problems with ouput when using df and getting strange results.

If I have a 10GB pool and I create 10 empty file systems, the sizes reported in df will be 100GB. It's not quite a lie either, because each of those 10 file systems does in fact have 10GB of space available I could write 10GB to any one of them. If I write 1GB to one of those file systems, the "size" and "available" spaces for the other nine will all shrink despite not having a single byte of data written to them.

With ZFS and df the "size" column is really only measuring the maximum possible size (at this point in time, assuming nothing else is written) so it isn't very meaningful, but the "used" and "available" columns do measure something useful.