pre-comp used space is less than post-compression

Hello!

Can someone help me to understand why my datadomain pre-comp space is less than a post-compression space:

dduser@dd# filesys show space

Active Tier:

Resource           Size GiB   Used GiB   Avail GiB   Use%   Cleanable GiB*

----------------   --------   --------   ---------   ----   --------------

/data: pre-comp           -     2301.4           -      -                -

/data: post-comp     3672.5     3410.5       262.0    93%              0.0

/ddvar                 78.7       15.8        58.9    21%                -

----------------   --------   --------   ---------   ----   --------------

* Estimated based on last cleaning of 2018/10/30 06:41:36.

Thanks.