Strava

A friend has huge scores for rides of similar intensity (as judged by overall time, and time in zones) compared to very small scores for me. I understand that his zones will be different, so here is an example:
FRIEND:
time on ride: 2hr 38, z1: 5m z2: 57m, z3: 57m, z4: 35m, z5: 1m, relative effort score: 238
ME:
time on ride: 4hr 24, z1: 33m, z2: 129m, z3: 70m, z4: 29m, z5 2m, relative effort score: 107

I know these scores are personal and are not to be compared between people, but really I'm interested in why it is happening and if Strava is aware of it.

Many thanks in advance.

Just submitted this to strava support, intrigued about what they say.