Plural or singular noun?

One previous bit of Q&A advice on the Chicago Manual of Style website noted the use of "Leon ... was united with Asturias from the eighth to the ninth century," concluding that "to" did not impart plurality, hence the use of "century," rather than "centuries." I realize that "eighth to the ninth century" is probably interpreted here as a shortened form of "eighth century to the ninth century."

But how can one justify "The first period covers the ninth to the thirteenth century," rather than "The first period covers the ninth to the thirteenth centuries"? Certainly a period spanning multiple centuries requires the use the word "centuries"? Or doesn't it? Am I missing something?

I'd very much appreciate your urgent advice (a deadline looms).

Many thanks,
Don

Email Removed