Substracting Two Measures

Hi,

I have two, relatively simple sum measures that produce two sets of counts.

Measure1

100

250

10

Measure2

1500

1000

50

Then I have a simple subtraction measure, Measure2-Measure1. This produces some correct results, however sometimes it produces very incorrect results. For instance, given the above it may produce

(1,500 - 100) = 1400

(1,000 - 250) = 800 ???

(50 - 10) = 40

It should be very simple, but it's not producing the right result. I've tried setting Measure1 and Measure2 equal to a var name, then subtracting those, same issue. I've also tried just shortening the equation to the measure by itself to confirm that it's producing the right counts going into the measure (so 1,000 and 250) and that is fine as well. It just doesn't subtract them correctly about 1/5 of the time. 

Any ideas?