Because there are other things that go into healthcare that are accounted for in NHS funding such as hospices and elderly care. The 83% in your link is the gov

Because there are other things that go into healthcare that are accounted for in NHS funding such as hospices and elderly care. The 83% in your link is the government spending on healthcare as % of total healthcare expenditure not the NHS budget as % of total expenditure. The government funds other healthcare related expenses that aren’t direct NHS funding.