The myth of the 180 day non-rule has been debunked here on travelexchange however there is no explanation if it is perhaps an unwritten one.
We find that there are many rules and hidden requirements including funds parking and provenance of funds in granting UK visitor visas which are not explicitly spelled out. We also know that there is nothing explicit in the rules that say a visitor cannot spend more than 180 days in the previous one year in the UK.
The rules do say:
V 4.2 The applicant must satisfy the decision maker that they are a genuine visitor. This means that the applicant:
(b) will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home;
Is the 180 day non-rule actually a defacto hidden rule & viewed as a violation of the above? In an article about the detention and denied landing of an Australian man to the UK, it explains the reason for his refusal as:
As a regular visitor to his family in Britain, he had already spent 180 days in the UK in the previous 12 months, and was classified as having overstayed his visa.