[SandWMan]

So, one in six are totally brain dead morons...I figured the ratio would be much higher...

[a fool in paradise]

So, one in six are totally brain dead morons...I figured the ratio would be much higher...

You can count on it.

[Clara Lou]

One in six actually believes that you can get something for (near) nothing. One in six has not heard or has forgotten “if it sounds too good to be true, it probably is.”

[Stoat]

...I figured the ratio would be much higher...

The article is written in such a way that it suggests at least some of the data was derived from people's responses to questions, which suggests that these are only the people who will 'admit' to acting on a spam mail.  From the article:

 

Eight hundred consumers in the U.S. and Canada were asked about their computer security practices habits as well as awareness of current security issues.

Those who did admit to opening a spam message -- which in and of itself could potentially harm their computer -- said they were interested in a product or service or wanted to see what would happen when they opened it.

So, the 'actual' percentage of people who act on spam mails may indeed be much higher, as you suggest....the rest are too embarrassed of their own stupidity to admit it.