The refusal reason given by the ECO is that bonuses and overtime is not counted towards income.

The Immigrations rules under Appendix FM (top of page 13) clearly states that
Overtime, commission-based pay and bonuses will be counted as earnings from salaried employment.
Well that's my understanding. Unless someone else has a different angle on it ?
To me it seems clear.
I guess the only 'weasel word' is "will be counted as earnings from salaried employment."