plural? singular?

[heidi95]

More than 28 million Americans suffer from migraine headaches. Their frequency and severity varies from person to person, but they strike women three times more often than men. And if there is a history of migraine in your family, there is an 80 percent chance you will have them as well.

Hi~
In this phragraph above, I think that verb should "vary". Because the subject is plural..(frequency and severity)
But I can see that phragraph in the web page, Foxnews.com.
Am I right or wrong? Please let me explain.

 

[stuartnz]

I am not a teacher, but I read that as two singular subjects rather than one plural subject.. "Their frequency varies, their severity also varies". The way it is worded in the article is the way it would normally be worded in NZ English also.

 

[2006]

More than 28 million Americans suffer from migraine headaches. Their frequency and severity varies from person to person, but they strike women three times more often than men. And if there is a history of migraine in your family, there is an 80 percent chance you will have them as well.

Hi~
In this phragraph above, I think that verb should be "vary". I agree. Because the subject is plural..(frequency and severity) Yes, "two singular subjects" make a plural subject.
But I can see that phragraph in the web page, Foxnews.com.
Am I right or wrong? Please let me explain.

2006