Depending on how you calculate it, you can get quite a bit of difference for the same person. With the weight difference between you, that is only going to be a wider range of values, so this seems as reasonable as any of the formulas I have seen.
I calculate calories in three different ways. 1) let the GPS do it. 2) Calculate using the standard met formula with a mod to take hills into account (my own creation). 3) use one of the many formulas to base calories on heart rate.
For an 18 mile ride on gravel with 1000 feet of climb, I get values ranging from around 1000 to 1400 calories using these three methods. Since this is more informational for me than anything I'm actually using to eat by, it does not bother me.