Some promos have a fixed odds offer that doesn't seem to change, some just use a % of current odds.
It's really about what the odds boost is offered for. It is very noticeable which books look for boost offer bets that they think will lose, compared to books who look at them like bonus money for loyal players and often boost stuff they think is close to a lock to win.
The general boosts on parlays do suck me in to play parlays when I had no intention to.