Hi!
Thank you for your comment!
It might be the case that we see three different factors that enhance each other. Mark already pointed out that a lot of people use garmin head units, and multisport users might be even more inclined to do so given that Garmin offers a wide range of devices for sports other than cycling (mostly running, I would guess).
Also, GC might not be as well known in triathlete communities as in cycling communities.
And thirdly, you are also correct that there are significantly less powermeter-users in the segment of multisport users:
So, 10% of Singlesport users do not have a powermeter, compared to 27% of multisport users. And GC seems to be not as popular with non-PM-users than Garmin Connect, and this will affect this question as well.
As you say, this is a correlation. It might be that the reason why multisport users tend to not use GC as much as singlesport users is in fact that they tend to own a PM to a lesser degree than Singlesport users.
From a user research perspective, I would not rely on quantitative data to infer any reasons why or how people are doing what they are doing. Quantitative data is a first orientation, the beginning of a map that can show you the way.
all the best,
peter