Yes, this seems to be broken on Strava now. EDIT--it only started doubling the calories in mid July 2018.
I have a power meter that does give me consistent results.

For a recent ride recording:
Strava shows:
1164 kj
2815 calories!! uh, no.
Golden Cheetah shows 1159 kj.


From a ride in Oct 2017:
Strava:
1003 kj
1118 calories
Golden Cheetah shows 997 kj.

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From one of the many explanations via google:
Kilojoules for a ride are quite close to calories for the the ride. (Calories might be slightly higher than the kj number).

Essentially:
about 4 kj is one calorie.
but people are about 25% efficient in converting food into useful work. (The other 75% is wasted heat!)
so 4 kj *.25 is effectively 1 calorie.