It can still be random and adjust the odds of jackpot trigger per bet size. The reason they’d do this is because the bigger bets contribute a bigger share to the jackpot – it wouldn’t be fair for someone betting a penny and contributing next to nothing to have the same odds as someone betting £25 and contributing a big share to the jackpot. I guess the odds of winning a jackpot are in line with the % share that they contribute.
I’ve won four of these, on the old Marvel jackpots (which Age of the Gods replaced). Three of the lowest, one of the second highest. Years ago now and still to this day one of my biggest ever wins my a mile.