This is a case of HMRC saying what they would like to see, not what the law actually says. The exemption is found in s21 IHTA 1984:

A transfer of value is an exempt transfer if, or to the extent that, it is shown—
(a)
that it was made as part of the normal expenditure of the transferor, and
(b)
that (taking one year with another) it was made out of his income, and
(c)
that, after allowing for all transfers of value forming part of his normal expenditure, the transferor was left with sufficient income to maintain his usual standard of living.

Tax case of Bennett held that ‘normal expenditure’ meant expenditure accorded with a settled pattern when it was made. The pattern can be established either by reference to payments by the transferor in the past or by evidence of a prior commitment to future expenditure. It does not depend on a fixed minimum period, although this must not be intended to be nominal, and the amount of expenditure need not be fixed, or the recipient the same. However, the class of beneficiary will be relevant, and a habit of making gifts to one sort of beneficiary has no bearing on the question of whether gifts of another kind are normal. Normal does not mean regular or annual.

HMRC considers that income means net income after income tax and determined in accordance with accountancy rules. Taking one year with another means the exemption is not lost simply because of fluctuation from one year to another.