The total amount of money available to pay out the prizes in the top two Set For Life tiers is limited to a certain amount. If the number of winners in either of these two prize tiers is unusually high, there may not be enough money in the prize fund to pay them all the advertised amounts. If that happens, each winner will receive an equal share of the capped prize fund.
You can find a more detailed explanation about prize capping on the Prizes and Odds page.