When clicking on someone’s avatar, one is taken to /u/:username which lands on the Summary tab. This seems logical to me.
But the g,p (jump to profile) goes to the Activity tab instead… Is this by design? I expected the Summary tab to be the logical place to visit a profile, even my own, as it it shows stats, top replies/topics, etc.
Yes, as it mirror the behavior of clicking on user cards:
That makes sense — thank you for explaining. Feel free to close/delete this if appropriate.
Is there a reason why clicking your own user card takes you to activity and not to the summary? Every time I click my own user card I am confused because it does not behave like the one of everyone else
I suspect it assumed the logic is that by doing so (clicking our own avatar) we are more interested in what we are doing rather than who we are. 
but that is only a guess. 
But when you use the notifications menu to visit your user menu then you are interested in who you are? Maybe that is why I am confused when I use the user card