people assume a lb of muscle = same amount of calories as a pound of fat.

it is not. ie 450g of fat = approx 4000 calories. the same amount of meat does not have anywhere near the same amount of calories.
ie 450g raw steak = approx 600 calories, obviously it would take extra calories to build the muscle so actual calorie usage would be higher than just the actual calories contained in the muscle itself.

I could be completely wrong here, someone correct me if I am, I just don't believe a lb of fat and a lb of muscle are the same, nor that it takes 3500 calories to gain a lb of muscle

I gained as much as 4-5lbs a week eating 1000 calories over. i was also getting 400g protein a day