How swimmingStrokeCount is counted? (both arms or one)

Hi! I'm making a workout app for swimmers. Our QA is confused with the swimmingStrokeCount output. He's a sportsmen and when crawl he used to count 2 strokes per cycle, one for each arm. The output is incorrect and looking at numbers we suspect that HKWorkout counts only one arm.

Documentation doesn't say anything about it.