How are you calculating the number of calories you are burning? HR monitors and Garmins often over estimate by a lot.
Of course, you might be * a lot* younger than me, and most people are bigger than me, so they burn more calories. But, at 5 foot 1 inch and 106 pounds, I don't count on burning more than 300-400 calories an hour, doing hilly riding at an average of 14-16. For me, I make sure I've eaten/hydrated right before a ride and I use only Nuun (electrolytes) in my water bottle, plus one bottle of plain water. I take the Nuun tabs with me, if I need to refill. I think i've used the lower cal Accelerade once this season and have bought 1-2 G2O drinks en route. If it's 25 or under, I don't need to eat. Above that, one bar suffices for 50 miles, with sometimes a Shot Block near the end. I have to eat a lot right when I'm done, though. If I do a metric, I might add in a banana,or another bar, preferably, real food. The one century I did, I ate every hour, small amounts. I brought my own whole wheat bagels and soy nut butter, along with the bars.
Weather does affect all of this. I eat more when it's cold.