ECM change and MPG

I recently purchased a 2006 Jetta TDI and shortly thereafter the ECM failed. I replaced it with one from the junkyard with an immobilizer delete. I drove about 1k miles before the failure and it averaged about 40mpg. After I replaced the ECM it has been more efficient. My current (albeit short 120 mile) average is 50mpg. If that ECM was tuned for better economy would it retain the tune after the transplant? If not, what would you suppose is the difference? My driving is consistent and similar. The fuel is the same that was in the car. Could the monitoring system have an error large enough to give those numbers in error? I'm really curious.

Unless you are judging full tanks of fuel hand calculated over several tanks, you aren't comparing two similar events.