Water bill formats

[somerfriend.livejournal.com]

Does anyone know what the water bill readings are rounded off to the nearest 100?   EG on my water bill the readings show as 31900.   The last 2 digits are always 00, or in the history section, they are dropped off entirely.  

Also what units are these readings in.   Bill shows C.U.   What is that?  Cubic unit?  What does that mean?

The Water Department's page on the Somerville website doesn't seem to have related content for these answers.