Why can't we arbitrage ETFs?

I invest in a couple of ETFs in Europe and regularly I observe results such as:

• VUSA (Vanguard’s European S&P500 ETF) went up 0.41% today;
• the S&P500 went down 0.68%;
• the EUR/DOL went down 0.13%.

If the ETF followed precisely the index, it should have gone down since both the index and the EUR/DOL went down. This should mean that the market maker will force the price to go down tomorrow. Can’t we use this to arbitrage? (We could short sell the VUSA today, for example.)

I really think that my reasoning is wrong since otherwise we really could make millions like that. But I don’t know where I’m wrong.