The fees charged by mutual funds range widely, so it’s not surprising that returns for those funds vary, even if they’re passively following a benchmark.
What about returns by the four low-cost ETFs? They also vary in their performance, even though they’re roughly charging the same low costs and following the same benchmark.
Blame benchmark-tracking glitches and their lucrative practice of lending stock to big banks, a move that can paradoxically cause a fund that passively mirrors an index to, in fact, beat that index.