Category A is for both salaried and non-salaried employment... the only difference is how they calculate the income. However, if his income varies each month, you can use Category B instead, if you want to.
For non-salaried, they should add up the amounts he was paid over the 6 months, divide by 6 and multiply by 12 to get the total annual income for the visa.
From your figures, the total earned was £9,415.37. Dividing by 6 and multiplying by 12 gives £18,830.74 which meets the requirement using non-salaried income.
However, I seem to remember from your refusal letter that they said you didn't meet it because the lowest payslip was £1,275.
But this is the salaried income calculation, not the non-salaried... so if they used this amount, they would have got your income as:
12 x £1,275 = £15,300.
However, is he definitely non-salaried? As in, he is paid by the hour and does not have a fixed annual salary?
If he is, but they used the £1,275 payslip, UKVI were in the wrong with their calculation.