You used something from Dieting for Dummies and came up with a slightly lower number? What's 'slightly lower'? I mean, these are just going to be ballpark figures anyway. If it's close at all, then the fitday calculation is probably correct - enough.
I'm guessing that you don't really measure and count every single thing you eat. If you are meticulous about this, you'll find out that it is extremely (I mean shockingly) easy to eat around 3,000 calories. I went to a restaurant one night and had a fairly 'healthy meal,' eyeballed the amounts, came home, put the stuff in fitday and already, I was almost 3,500 calories for that day. The day had been low-cal enough to begin with!
Eat as many things as you can that have the nutrition printed right on the package or box (including fresh, washed vegetables), measure oils and butter, don't eat anything that isn't measured and truly the quantity that you enter in fitday and you'll see over a couple days how much you actually do eat. How easy it is to eat 3,000 calories!
If you're eating that much and losing weight - because that's your 'RESTING' BMR - then you can rest assured that's pretty much what your BMR is. You can even add calories to that and see what you can eat and still lose weight.