Kalshi's September contract settled on a trimmed-mean index from Sept. 1. Polymarket's relisted market counted only Binance prices after Sept. 23. Meanwhile, odds of a new September high collapsed.
Two contracts asking whether bitcoin would reach $85,000 in September both resolved yes. They did so four days apart, on different price sources and different windows, which shows why similar-sounding markets cannot be treated as the same bet.
On Kalshi, the $85,000 strike of the monthly one-touch contract settled on Sept. 21 at 14:31 UTC, with an expiration value of $85,076.37. Kalshi's rules count any time from issuance through 11:59 p.m. Eastern on Sept. 30 and use a one-minute trimmed mean of the CF Benchmarks Bitcoin Real-Time Index, which removes the highest and lowest 20% of prices across exchanges.
On Polymarket, a market titled "Will Bitcoin reach $85k in September 2026 (from September 23)" was created on Sept. 23 and specified that "price action before this market's creation will not be considered." It uses the high of one-minute candles for the BTC/USDT pair on Binance. It resolved yes at 7:33 a.m. Eastern on Friday, Sept. 25. Quotes of about 71% on that market seen earlier in the week predate the touch.
Why the rules differ
A trimmed mean across many venues is designed to ignore brief spikes on any single exchange. A single-exchange candle high captures them. A contract on the first can miss a move that triggers the second, and a contract that starts counting from a later date can require a second touch of a level that bitcoin already reached earlier in the month.
The repricing
The higher strikes moved sharply on Friday. Kalshi's contract on bitcoin exceeding $87,500 in September fell from 36% at 5 a.m. Eastern to 29% at 10 a.m. and 21% by 11 a.m., and traded near 23% to 24% in the afternoon. The $90,000 strike fell to about 8% to 9% from 11%. Polymarket's comparable $87,500 market traded near 24.5%, down about 12 points on the day.
The declines followed bitcoin's own path. On Coinbase, it traded near $84,570 at 5 a.m. Eastern, rose to about $85,250 in the 7 a.m. hour, fell to a low near $83,091 in the 10 a.m. hour and was around $83,800 by early afternoon. About $15.6 billion to $16.1 billion of bitcoin options expired on Deribit at 08:00 UTC, the quarterly settlement, which may have contributed to the moves around that time.
Demand for spot bitcoin funds has also been slowing. U.S. spot bitcoin ETFs took in $190.7 million on Sept. 24, the sixth straight day of inflows but the third straight day of smaller totals.
With five sessions left in the month, a price near 23% means traders see less than a one-in-four chance that bitcoin rallies another 4% or so to $87,500. For anyone holding the contracts, the settlement index matters as much as the direction: a brief spike on one exchange could pay out on Polymarket and not on Kalshi. The window closes at 11:59 p.m. Eastern on Sept. 30.