I've noticed great variation too. Start looking at the ingredients for some more info. For example, lots of breads contain high fructose corn syrup, and maybe that's adding calories. I've never mapped calories from bread to certain ingredients, though, so don't know for sure.
Another issue might be that some manufacturers don't count the calories from fiber, as it is indigestible. If you're looking at high fiber breads you may be able to see some discrepancies. What's the total number of carbs? If you multiply fat grms *9, carb grms *4, and protein grams *4, you should come up with total calories. If the total you come up with is a lot higher than the label, look at the fiber grams. If you multiply fiber grams *4 and subtract that from the total calories, now does it look like what the label says?
Or, I'm completely off base!!