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Connect your charger however you may have had an app running in the background which could account for this
I've seen that behavior before, it has happened to me, restart the phone and check again, perhaps an app was acting up, 2/3 of the times it has happened to me, it was a game or an app acting up and once I restarted the phone the battery was back to its original %, the other 1/3 it was a huge drop, but it makes sense because I was syncing a lot of data and playing games.