The case of the shrinking tape

I print decals for a company who uses them on products that they sell for measuring water levels. Essentially, they are millimeter rulers. I have set this up in Flexi AND Omega, both, using the rulers set to millimeters and they are right on on the screen, dead on to the measurements set up in both programs.
When I print this out to the Edge, they end up approx. 4 mm off? WHY!?!?!?! Arrgh! I'm surprised that it would be that much off, do you think it's caused by the heat of the printhead?
I kept thinking he was stretching them putting them on...not the case, right out of the edge, without being removed from the backing sheet, it's off.

 

Thanks you guys! I print stuff all the time, I have never noticed anything else being off; mags, decals etc. I am printing some decals today and I am going to print a box as long as the tape and see if it measures up.
I am thinking it has to be something wrong between the rulers in the computer and the output, rather than heat. Edge positive gives me the same results as the Oracal 651...
I set this up by using a line segment, repeating it using the repeat tool making them 2mm apart, then I added a stroke to the line segment. According to the rulers and measurements in both programs it's right. According to the output and a millimeter tape measure, it's wrong. The guy told us it was off by 4mm, I am finding it's more like 2.5 mm. I'm baffled!

 

Well, I believe the mystery is solved with the help of a Gerber Mastermind...(Thanks Tony T!)
Sprocket Holes! The measurement from the center to center of the 3 sprocket hole punch is suppose to be 5", but in actuality, mine varies. Over a 10" span the measurement can be off by .104" (2.6416 mm)
I am printing on Oracal (and we all know they have had their issues with the hole patterns) and I'm pretty sure Gerber would try and hold a tighter tolerance than that.
I am going to attempt to play around with this to get it sized fairly accurate but with the hole pattern being the issue, it's always going to vary from roll to roll. I wonder...are there certain brands that hold a tighter tolerance with the hole punch than others and would the clear mylar backing sheet help with hole tolerance also? Maybe I need to use Gerber vinyl with the clear mylar backing sheet to get the best results? Does anyone know what the number is for that stuff? Isn't it just the regular number with an 'i' added to it or something like that?

 

Thanks