Yield Management is a bit more sophisticated than that. £10 fares (which incidentally I have never encountered myself, and have only met one person who got one) are only offered on flights where it is not expected to have demand beyond capacity at higher prices. There may be an occasional mismatch here, but they are sufficiently infrequent that it's no real concern, or revenue impact, and any such are analysed to see if the calculation needs to be changed.