I'm just speculating now, but it might be having reactions with other cells in the lemon, causing the taste to change.
The only thing that is for sure is that the test of the juice drastically changes if kept for long (even if in the fridge), whereas it doesn't, if squeezed out beforehand and then kept in another container, in the fridge.
Or just buy concentrate from start.
I think I'll stick with fresh, even if frozen.