Mine has the same peculiarity-- I only did two rides with groups all year, and Strava gave me something like 9 for one ride and 18 or so for the other. Thing is, on the first one I rode the whole day with a group of about 25 (out of around 200 riding that distance) and in a pack of over 40 in the second ride-- and the 50 mile route that day had ELEVEN HUNDRED entrants.

I have also gotten a "rode with" with a complete stranger on a normal solo day, where we covered the same roads about 60% of the time, but never saw each other.