Subtracting Columns from Two Related Tables - Valu...

I have a scenario where I want to subtract a fixed monthly target from a dollar figure, each month, for a series of contracts. I have a related table that defines the contract number and the fixed target I want to use each month. When I create a table showing the contracts, useage dollars, month and target from related table, everything looks as it should.

The problem begins when I add in the column as a RELATED column. I did this so I can make a quick calculated column to show am I above or below target. What I am seeing is some inconsistent behavior where the related value added is multiplied 10X or up to 12X the original number.

My question is simple, why is the behavior happening and what is the best way to subtract two columns from seperate, but related columns? Adding in Related, then doing quick Calculated Column OR calculated Measure.

Any and all advice is appreciated.