I disagree with your premise. "Last Monday" is the closest Monday to today going backwards. If today is Monday then "last Monday" is exactly a 7 days ago.
Since this is the internet I could just say "You are wrong" and leave it there
I don't know where the value of "last commit" comes from but I suspect someone at GitHub coded it and KoLmafia.us only displays it. So if there is a cultural difference it would be between you and someone writing code for GitHub.
As a clarification, today is Saturday February 8. I say "last Monday" is Monday February 3. I believe you are saying that since February 8 and February 3 are in the same week (since a week started on Sunday February 2) "last Monday" would be January 26. If that is a correct understanding then you are the first person I have ever encountered who would define "last Monday" that way. I live a sheltered life? But I have found people who use "last week" in a way that is confusing to me. Sometimes "last week" seems to mean "at least seven days ago" and sometimes it means "before the most recent Sunday". If it really matters there is usually continued clarification with reference to specific calendar dates. Perhaps I have encountered people with your definition and not realized it?
The cause is probably cultural. It might be region but it also might be generational or even occupational. People whose primary means of "telling time" was formed using an analog clock face talk about time differently, in my experience, compared to people used to digital displays of time. People who have to do calculations with time (accountants, programmers, etc.) are much more concerned with precise definitions - When does a day start and end? When does a week start and end? How does an employer honor a contract that says "four weeks of vacation" for employees who normally work four (ten hour) days per "week" or work less than forty hours "per week"? when pay is computed at an hourly rate? - than people who just "live" with time.