How are Wikipedia subcategories meant, semantically?

Let’s have a look at the category

https://en.wikipedia.org/wiki/Category:British_politicians

Theresa May is not in it. So let’s see:

• Theresa May is in Category:20th-century_British_women_politicians
• which is in Category:20th-century_British_politicians
• which is in Category:British_politicians_by_century
• which indeed is in Category:British_politicians.

So those categories seem to be inclusive: the items of a subcategory are thought to be in the parent category as well.

However, then we have the following problem:

• The 2019_Conservative_Party_leadership_election is in the category Category:2019_Conservative_Party_(UK)_leadership_election
• which is in Category:Theresa_May
• which also is in Category:20th-century_British_women_politicians

Uh, so this means that the 2019 Conservative Party leadership election is a British politician too?

So there seem to be two kinds of "subcategory relations":

• this category is a subcategory, all its items belong to the parent category (Theresa May –> British politician)
• this category is just related to that parent category (2019 election –> Theresa May)

Is that just messy, or am I missing something to distinguish between the two types?