To achieve that consistency, they tend to over-roast their beans -- to the everlasting chagrin of coffee lovers everywhere.
On a similar note, dunkin serves light roasts. I'd presume they also want consistency?
Presumably they over-roast it because the most popular drinks have a lot of sugars and creams (Frappuccinos, etc.) so the coffee needs to be stronger. So, it’s not the over-roasting that makes it consistent, but the fact that every branch roasts it the same way.
Dunkin on the other hand seems to make a serious portion of their income from those boxes and their light roast black coffee, so they maintain consistency with a shorter roast time.
Of course most coffee places have various beans these days and this doesn’t always apply.