It keeps on changing every week because every week as well winners will be announced. I think lottery has always to depend on BTC price on its prizepot and it's always $7500 (can't remember clearly if it has changed).

By the way was this a bug? I get a glance in it when I head for the lottery tab then click the WINNERS link just below the daily jackpot using my mobile and I find it odd that the winner was only the same UID is this a win streak of him? Please do check the image below.