Nat Narayan is a small section drawn from the later Gurus, and its heading names two measures together rather than one.
Compound and qualified headings are common in the volume. Some name a measure and a variant of it; others, like this one and like Mali Gaura, join two names. A musician reading such a heading understands it as a specification, either of a measure with a particular colouring or of one measure performed in the manner of another, and the distinction between those two readings is not always obvious from the heading alone.
The consequence is that counting the volume's rags is a matter of convention rather than of arithmetic, which is why the commonly given figure of thirty-one is a count of principal sections and other counts run higher.
Beyond the counting, the compound headings are evidence of something the Granth otherwise says little about: how precisely the Gurus specified performance. A tradition content to indicate a general mood would not distinguish Gauri Bairagan from Gauri Purbi, and would not name two measures together. The headings imply an audience of musicians who could act on that level of detail, and imply that the compositions were set with that level of care rather than assigned a measure after the fact.