How many calories burnt at 200watts/hour

By my understanding, the general rule of thumb is, on a moderate to hard ride you will burn 600-1000 calories (depending on weight, effort etc)

I’ve just done a 1 hour turbo session where I averaged approx 200W. According to the bastion of truth Strava, this means I burned 651 kJ. Various unit convertors on the interwebs tell me this is 155 calories.

So, my question is, where did all the other calories go? Did I really only burn 155 calories on what felt like a pretty hard interval session?

Thanks