An amount appearing under a heading you did not expect has four usual explanations.
The vendor was recognised as something else
Categorisation starts from who was paid. A supplier whose name resembles a common retailer is assigned by that resemblance until it is corrected once.
Correcting it on the transaction teaches the assignment for that vendor going forward, so the same correction is not needed monthly.
A rule is applying
Where a rule assigns transactions matching a description, it applies to everything matching — including transactions it was not written for.
It is a personal cost
Personal spending on a business account belongs outside the business figures rather than in a business category — see marking a transaction personal.
The chart of accounts has two similar headings
Where two accounts mean nearly the same thing, transactions divide between them and neither total is complete.
This is the most common structural cause and the one worth fixing rather than working around — see chart of accounts cleanup.
Why it matters beyond tidiness
A cost on the wrong line moves gross margin. Direct labour recorded as overhead makes margin look better than it is and overheads look worse — see reading a profit and loss.
After correcting it
Figures built from the category update on the next calculation rather than immediately.