If you pay for someone to care for the disabled person whilst working, the amount you pay or half of your earnings, which ever is lower, can be deducted when calculating the earnings.
Each benefit has its own conditions of entitlement and some are not payable as soon as the hours worked or earnings exceed a certain figure. In the case of Carer's Allowance, a penny over £102 earnt in a week will mean the benefit will not be paid for the following week (unless the pay has to be averaged. )
It may not appear to be fair but no matter what the limits are, there will always be folk who will not be entitled because their earnings exceed.