I don't get why Strava uses HR for calories when it has PM data readily available. One is a direct measurement of energy expended while the other is an estimate based on something that is influenced by a wide number of factors.

I had one ride where my HRM was acting up and reporting HRs in the 60-70 range. The ride was ~800 kj but Strava says I burned something like 6 calories. Then, as others have said, on hot days (and really just in general) the estimated calories are significantly overestimated.