From gov.uk - the bolding is mine, as being the parts that might explain the above. I presume that 'unknown' is because sex was not provided at some point in ygd data collection procesz.
Police powers and procedures: Stop and search and arrests, England and Wales, year ending 31 March 2023
https://www.gov.uk/government/statistics/stop-and-search-and-arrests-year-ending-march-2023/police-powers-and-procedures-stop-and-search-and-arrests-england-and-wales-year-ending-31-march-2023
"2.6.3 Sex of persons searched
In this report, we refer to sex rather than gender of people stopped and searched. ‘Sex’ can be considered to refer to whether someone is male or female based on their physiology, with ‘gender’ representing a social construct or sense of self that takes a wider range of forms.
The data collection gives 4 options for the sex of the person searched: female, male, other and unknown. ‘Other’ may include people who define themselves as non-binary or transgender. However, only 20 of the 44 police forces recorded any stop and searches of people defined as ‘other’ sex. This suggests that different recording systems used by police forces have different levels of detail to which the sex of the person searched can be recorded. Additionally, it is likely that recording includes a mixture of physiological and personal identity.
The following analysis only includes records where the sex of the person searched was provided and excludes vehicle-only searches."