Probably because your datastores are over committed. If you select thin provision when you build a VM it only writes on demand, so you can tell a VM it has 100 GB of storage while it only uses 10 GB. This adds up so say you have 40 TB of datastore space and only 30TB is used, but if all the space those thin provisioned disks filled up you might be at 50TB of data (10 TB over committed). Thin provisioning is good if you keep an eye on it and always keep a little space on your SAN free to add an extant if needed, but can get you intro trouble if you let it fill up.
So usage is the actual space used by thin provisioned disks. Commitment is space provisioned to VMs that they could use if they filled up their thin disks. Since you're over 100% committed, be careful. Keep an eye on it and plan to buy more storage or clean up what you have.
