is not exactly correct, but captures the main point.
Numbers are made up to illustrate the main point. Let us say you normally double 9 v 2 at +1 (normally .25% edge for the 6decker in question) , with a $50 bet, and a 1% edge on the doubled money. No problem putting out an extra fifty on a 1% edge.
Now 2-1 promo and you get a 9 v 2 at +1 but now you have $250 bet.
Do you wanna put another $250 at 1% edge? Probably not if you have a 10K roll. so index goes up.
That is a characteristic of ra indices that they change with bet schedule.