They’re not really very accurate, unfortunately. Some of the newer heart rate monitors (Polar, Garmin, maybe others) are better, but they’re still not great. The only real way to get an accurate number for calories burned is to measure your power output with a power meter, then convert the energy you expended (in kiloJoules) to calories.

To put your calorie numbers in perspective, on most of my rides, I’d ride a 20 mile ride similar to what you did in about an hour at about 20 mph, and burn 700 calories or so while doing that. A rule of thumb that I use is that each 100W of power output averaged for an hour is about 350 calories burned.

At the speeds you’ve mentioned, your probably putting out an average of 50-70W, so you’re probably burning about 200-300 calories per hour, a lot lower than the almost-1000 calories you posted.

And just because a ride felt easier doesn’t mean you burned less calories on it. Climbing hills can tire you out much faster because you’re pushing yourself harder. Back to using normal power output, if you normally ride along at 12 mph and put out 50W while doing so, having to put out 100W for five minutes to climb a hill is going to make you feel a lot more tired than riding for 10 minutes at your normal 50W, but the calories burned will be the same. A long seemingly-easy ride that you feel like you can do every day can burn a lot more calories than a short, hard ride that makes you take a few days off.