It’s my understanding that Starbucks universally over-roasts their coffee to keep it consistent. Normally the roast time would depend on each individual batch.

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.