Chris Webb looks into activity IDs:
I was looking at the output of Power Query’s Query Diagnostics feature recently (again) and trying to understand it better. One of the more confusing aspects of it is the way that the Power Query engine may evaluate a query more than once during a single refresh. This is documented in the note halfway down this page, which says:
Jorge’s comment on the post adds even more context around what the ID Chris comes up with actually means.