Starbucks owns the trademark FRAPPUCCINO, one of its most popular drinks brands. The BLA decided that Frap was similar because it constitutes a prefix or dominant part of Starbucks' registered trademark FRAPPUCCINO.
The containment of a trademark entirely inside another is a regular headache for IP offices. The extent to which added material in the senior mark renders it different is not usually considered. Although here the focus on the prefix might be right given Starbucks' long and widespread use.