When I go to my own reputation page, which has several different display modes (by post, by time, graph), it will default to whatever my last choice was. Same with activity, favorites, etc.
Other tabs default to something specific: the Badges tab seems to default to "all" and "General"; the Tags tab to "popular".
When I click on the Users tab, though, and it defaults to the "Reputation" display, sometimes I'm shown the "Week" total, sometimes the "Month" total, sometimes the "Quarter" total, etc.
I noticed today that it was showing me the Quarter total; I also noticed that I am listed first in the Quarter, Yearly, and All total, fourth in the Monthly total, and second in the Weekly total.
I'm guessing that the timeframe it defaults to is the one in which I'm ranked highest, with ties being broken in favor of the "shortest" timeframe. Is this correct?
Update. Well, my guess is definitely incorrect; right now I am being defaulted to the "Week" view, which is not my highest ranking. I wonder how it decides what to default to?