Sets use Maker and Chainlink oracles in order to get on-chain price data for rebalance auctions. A price mismatch is when the real time price data pulled from CryptoCompare deviates from the oracle's current price enough that it has a noticeable affect the rebalance auction.
Rebalance auctions for Social Trader Sets aim to start at 1% away from fair value. If the mismatched price is far enough, it may start the auction past fair value which may potentially increase the amount of slippage a rebalance will incur.
You can either start the rebalance with the mismatched price knowing that there will be a higher likelihood of slippage, or wait until the oracle's price is updated to the correct price.