Surely if benefits are part of your income, a £0.63 reduction in your benefits is a £0.63 reduction in your total income (up until the benefits are zero)? In other words, an effective tax rate of 63% on earnings while you receive a nonzero amount of benefits?
This prevents situations where it's better to stay under a cutoff of X in income until you can jump straight to an income of X income + Y in benefits, while staying on benefits until then.