I don’t think the disabled child premium gets you any earnings disregard.
So applying the formula D = MIN(£20, C£e+P£e, PD), the system has correctly identified £10 as the disregard.
C£e = carer’s earnings, in this case zero
P£e = partner’s earnings, in this case more than £10
PD = disregard that would otherwise apply to partner’s earnings, in this case £10.
