[TeMPOraL]

The author covers "unknown unknowns" explicity;

> It’s interesting to note: (...) the amount of work “outside” the project, aka unknown unknowns

If I understand correctly, most of those fall under the blue "Problems" category, aka. "the work outside the work" - though there may be some bleed-through to the green "Iteration" category, aka. "the work between the work", in which the author includes "debugging, refactoring, maintenance, tooling".

This is valid by my experience too; most of the schedule-breaking "unknown unknowns" fit into the "Problems" category.