From what I gather, it weighs champions you play heavier, not taking into account whether you already own skins for them. It also takes a -large- amount of games to get a proper sample size for them to work with.
For the Snowdown sale, I had been away from the game since the previous Jan/Feb because of a faulty mouse, and only just returned this November.
I was offered:
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Mafia Jinx at 40%: when I was testing out my new mouse and settings, I used Jinx in a couple custom matches. As an ARAM main, these were the only games I've played on SR in over a year.
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Panda Annie at 40: I do have 2 Annie skins already, and I used to try to trade for her in ARAMs.
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Debonair Vi at 50%: when I do Rift, I'm a Jungle main, and I don't always trade/reroll Vi in ARAM because she is useful, but I got the Neon Strike bundle when she released.
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Sandstorm Ekko and Ekko at 50: This one I think was a filler offer. I don't play assassins, and I never played Ekko before (still haven't).
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Headhunter Yi at 70: another mostly filler. I attempt to trade away Yi in every ARAM lobby, and I was much more of a Trynd/WW Jungler back when on-hits were good.
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Amethyst Ashe at 20%: another partial filler I think. I already have 3 Ashe skins (ONE OF WHICH RIOT FORCED ON ME, IF I COULD REFUND HEARTSEEKER I WOULD BUT THAT WAS THE FREE SKIN THEY GAVE ME LAST TIME). I liked playing her (her rework is iffy for me, really requires a competent frontline to hold the enemy back long enough to actually activate her Q), but even before I stopped playing for a year I was much more likely to play Jinx, MF, or Graves in botlane.
With Poro King done and over though, I have champions I've personally picked to play in my recent match history, and the Lunar Revel Shop is MUCH more indicative of what champions I like to play.
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Hextech Janna @20: I didn't play her during the event, but I've somehow rolled her 8+ times in the past week in ARAM. Even considering how many games I play and the size of my pool, that's still unlikely odds.
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Master Arcanist Ziggs @70: Looooove Ziggs, he was my goto mage during Poro King.
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Cottontail Teemo @30: I adore this little furball of hellish nightmares. Easily my most played champ during Poro. Already have Badger Teemo.
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Barbarian Sion @30: Aaaand here's my favorite champ. Second most played during Poro, and I have no skins for him yet.
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Marquis Vlad @70: The second champ I ever played, waaaaay waay back. Recently unlocked him though, always put it off.
Also, when I opened the shop this afternoon, my first reveal was Mafia Jinx at 70% (the same first one I got at Snowdown, just cheaper). But, when I opened it again just now she was replaced by Piltover Customs Heimerdinger at 70%, another champ that I used heavily during Poro King.
All in all, I was very surprised by the turnaround compared to Snowdown. The list accurately represents my playstyle and champion choices.
