It's all very interesting. I've found some references now. Woohoo!
Under the topic of "notional agreement" where the grammatical rules would indicate a singular, but the intent would be plural. In these cases, the verb can sometimes agree with the notion/interpretation rather than the subject. I found this:
Expressions of the kind one in five, one in ten and so forth can also take either grammatical or notional agreement –
‘Just one in five Britons eats the recommended five portions of fruit
and vegetables a day.’ (Grammatical agreement)
‘Just one in five get ‘five a day’.’ (Notional agreement)
The case for grammatical agreement is clear enough. The subject one takes singular verbs: one gets, one eats. If we delete what comes between
the subject and verb, therefore, we get –
Just one eats the recommended five portions.
Just one gets 'five a day'.
The case for notional agreement rests on the fact that both statements are obviously referring to multiple groups of five. Given a total of 60 million
Britons, then, the number of people eating their five a day is 12 million, and 12 million people eat and get, not eats and gets –
Just one in five eat...
Just one in five get...
This is from a website called "Grammar and Style in British English". I'm not saying it's THE authoritative source for grammar rulings, but it's possibly a bit more precise than a forum discussion - at least for British English.
Sorry - I know people think this is all a big argument over nothing, but I actually find grammar can be quite interesting, so once this came up I started exploring. I'm surprised I don't have a few grammar books on my shelves.
I've also seen another site refer to the construct of "1 in 6 ...." as a noun phrase, which sounds reasonable, but when substituting a pronoun you could still substitute "he or she" as opposed to "they". So it being a noun phrase doesn't really solve the problem of agreement.
So basically, "notional agreement" would happily explain the plural usage. As to whether you'd agree (pun intended) that the case for notional agreement is such that it should override the formal agreement rules, it looks like it's been left a matter of taste, at least in the site I referenced.
Incidentally, in spoken usage, I would probably use the plural. English is fun, isn't it?