The QRH is only actioned when the light comes on irrespective of whether its 453 or 907 aside. Its triggered by a wing tank reaching one of those figures not a total amount although in practise 907 or 1814 will be the least you have when the first light comes on.
As to why its one thing for ETOPS and another for non ETOPS I can't tell you I'm afraid. No experience of ETOPS to draw on so I can't even hazard a guess. Maybe its something to do with reserves in order to be ETOPS at the time? as 907 very roughly equates to final reserve. What I can say is that below a set amount of fuel you have to switch the electric hydraulic pumps off on the ground, I think its 1400 there or there abouts, due to cooling and it does seem a touch odd that the low fuel warning is not set at or above this with a QRH ref to remind you to switch them off.