> For instance, in table 3, it looks like they excluded backend tasks {0,1} (for frontend tasks {0, 1}) then {2,3} (for frontend tasks {2,3}) in the N=10 case, but backend tasks {1,2} then {3,4} in the N=11. Why the discrepancy?

With N = 10, there will be N mod k = 10 mod 4 = 2 leftover tasks, and so the round-robin fashion excludes {0, 1} then {2, 3}. However for N = 11, there will be N mod k = 11 mod 4 = 3 leftover tasks, so the round-robin fashion excludes {0, 1, 2} then {3, 4, 5}.

But as joatmon-snoo correctly said, the more important point is demonstrating how bad backend churn is with this algorithm.