Let's say I am targeting "invest in the world" with VT as my benchmark (currently 66/34 US/Intl). Let's say I have an account that is 66% FZROX and 34% FZILX. Over time the account will shift toward FZROX or FZILX and the benchmark will shift too. So I rebalance (when I remember).
But why does this happen?
If I'm "investing in the world" and the US grows relative to international, shouldn't FZROX grow and also the US allocation of VT? Or the opposite? What's the mechanism by which the investment stop matching the world?