HRM calorie burn accuracy

Couldn't find anything when googling this group, but I'm sure this must
have been asked here before.

I had a very enjoyable 100+km ride today in the "spring" sunshine (what
happened to winter? I must have blinked and missed it) wearing my HRM as
usual.

It claimed I used 4600 calories. This seems excessive to me. I was
riding for about 4 hours all told, at about 75% of my Max HR (avr 140).
I would describe my cycling as "moderate" - not a tough workout by any
means. Most websites I've been to about calorie consumption would put a
rate of about 500 calories an hour for "moderate" cycling.

Ergo, I would have expected to use about 2000 calories. Are HRM's just
that approximate?

--
Chris