Estimate calculation different than Part Cost

Hello, 

I have noticed an issue regarding the Estimating module that I do not understand. It appears that when bringing in an Existing Inventory Part and running a Calculate on the Estimate, the unit cost does not match the Part Cost in IFS. Sometimes it is off a few cents, others it can be more than $1. 

Below is a screen shot of a single Routing Op step part, in a single Site, with a basic Product Structure. 

Running Calculate on the Estimate shows a difference in the total cost:

The Routing and Product Structures all match in the Estimate. Any guidance would be greatly appreciated!