I was taught that the default is to write out numbers, but if you’re comparing multiple numbers, they’re normally supposed to be written in numeric form. I feel like they should have either started with a number or restructured the sentence.
googles
Apparently AP style guidelines say that for ten and above, you should use numeric form. Below that, write it out. That may be the driving factor here.
In general you should spell out numbers one through nine in AP Style. Consider the following examples of AP Style numbers,
The Chicago White Sox finished second.
She had six months left of her pregnancy.
You should use figures for 10 or above and whenever preceding a unit of measure or referring to ages of people, animals, events or things. Also use figures in all tabular matter, and in statistical and sequential forms.
I generally agree with most press conventions, and I’d buy into some of that, but I don’t think I really like the “ten cutoff” convention.