the variance in them is insane and multifaceted
you have all the normal variance such as when you go all in what the result is, but then there are layers of variance on top
prize pool variance.
you can run below EV in spinning non 2x prize pools.
you can also be unlucky and run bad in the high prize pools that you do end up in
you can run bad in the number of btns you are dealt into. you wont have an equal number of BTN, SB and BB in any given playing period. the BTN is by far your most profitable position so if you end up playing fewer buttons this will reduce your winrate.
you can run bad in the players you are paired with. the difference in winrate v recreational and regs will be wildly different so having more 2 reg games rather than 2 rec games will reduce your winrate.
all these can compound on top of each other. so you can spin more than your fair share of 2x games and then get lots of regs, few buttons and run bad in card distribution and allin variance in all your higher multipliers.
downswings are unavoidable in these games, 100 buyin downswings will happen
you can model the swings using the excellent free swongsim prrogram:
https://forumserver.twoplustwo.com/168/free-software/swongsim-roi-variance-simulator-lottery-payout-sngs-2k-post-1468580/i stopped playing them last year after the rake and structure changes.
if you want me to glance over your stats, especially your heads up section i might be able to give you some pointers, depending on what stake you play [im a former midstakes player, now playing hu casually at low stakes]