They cite a study in the Journal of Analytical Toxicology (from 2006) that found that



They also tested 6 cups from the same Starbucks (one in Florida) and found 12mg/16oz.

The article also references the recent Consumer Reports article testing of 6 cups each of 6 different brands of decaf.  Here's an interesting statement by Consumer Reports:

One of the six cups (12oz) from Dunkin' Donuts had 32 mg; one from Seattle's Best had 29 mg; and one from Starbucks had 21 mg.

What's weird about it is that Starbucks actually tells you there are 20mg in a 12oz cup of decaf.  So why did only one of the tested cups have that much?  Keep in mind, the toxicology journal article found an average of 9mg/12oz at one store,  6mg/16oz in another store.  We can assume the beans are the same, so the answer, obviously, is in the preparation of the beverage, not the decaffeination process.  Specifically, temperature and time.

Have you noticed that Starbucks and Dunkin Donuts coffee is so hot it makes you want to punch a harp seal?  And do you remember when McDonalds got sued because some damn fool used coffee as an eyewash and burned her corneas out? 

And have you noticed that the coffee you make at home is never as good as the one at the store?  And never as hot?  And never as good? 

Professional coffee makers heat the water that goes into the grounds to F195; your countertop bling-bling does 180.  Think 15 degrees doesn't matter?  

The hotter the water passing through the grounds, the more caffeine; but the more acidic.  I'll say it backwards, too: the colder the water, the less caffeine, but less acidic.  Did you ever wonder why no one ever orders Starbucks black?  It's because they're all dead.  If your machine isn't pushing 1200 watts, it's not even trying to extract caffeine from the coffee.  It's giving it a gentle misting, like you might to do to a baby, or when cleaning the leaves of a rubber plant, or when peeing on a friend.  Or something.  

Also, the last ounces out of the grounds are more bitter than the first.  They also have less caffeine.  You know those auto shut off coffee pots in hotel rooms, that stop dripping when you pull the carafe (and consequently flood because you didn't know you needed to put the cover on to get it to work?)  Each cup you pour is different in taste and composition.

That said, coffee on a hotplate burns fast, very fast.  So drink fast.

Also see:

What's The Healthiest Coffee To Drink?