Returns from slots are calculated over the long run. While spinning 10 million times will get you the exact percentage return stated for each game, short gambling sessions will see huge swings of fortune. Slots with progressive jackpots take part of the price for each spin to add to the big prize pool. This means less money is available for the regular prizes.
On average, most people playing will get smaller returns from progressive slots. When you add in the small number of players that win the huge prize, the returns are in-line with other games. Jackpot slots are not tighter than regular slots though if you count the returns over long periods.