Do we burn a lot less than we think

I just recently purchased a real heart rate monitor, when I say I mean one with a chest strap instead of one with electrodes that measure with you finger tips. I took the HRM out for a spin in Master swim class on Thursday night and after 60 minutes of swimming about 2000 yards, my watch told me that I had only burned 403 calories! What the heck? Seemed really low to me, it even seemed low to the swim instructor.

Today, I had a weight session, 20 minutes on level 10 on the stair mill and 10 minutes on the rowing machine. My HRM said that I burned 250 calories. And get this, the stair mill also keeps track of your HRM and calories and it said that I burned twice as much as my watch.

I'm trying to figure out what could be the issue. For the stair mill, I did not program in my weight, thus it using the default weight which is probably 150 pounds. Another issue is, I did not program in my max heart rate into my watch, so the watch is also using default settings. It had asked me to enter in my age and my weight and then set target heart rates for me - I'm guessing that it has my resting heart rate at 75 BPM. However, my resting heart rate is closer to 55 BMP. So what I have done is set my age to 20 to see if that makes any difference.

All this calorie counting has made me think twice about how much exercise I need to do to maintain my fitness. Today, I found myself counting calories which is not a thing I enjoy doing.