How Does R31804 Work with Standard Costing and Backflush Parts & Labor?

We have been using standard costing (07) and we backflush all our parts and labor for every WO in a few branches of ours. In previous years we have had a small variance sum in our 5090 & 5290 accounts because we backflush parts and labor to match the planned parts and labor to a "T".

This year we are seeing a VERY large amounts in those accounts and I am not sure what would have happened? Would us doing cost rolls every month instead of every quarter cause this? We are confused how a variance would happen if the planned amount parts & labor = actual amount parts & labor?