120% Sugar
The cold bar at Starbucks is where all the blended drinks are made. It’s also where the sugar-laden concoctions are prepared. Anyway, if you order a certain drink with a base that’s not available, this employee isn’t going to call you back to order something else. They’ll take the lazier route instead.
What they’ll do is use a completely different base instead, which, according to the OP, is pretty much indistinguishable from the other bases. Why? Because it’s all just 120% sugar. The flavor of the actual base is masked by so much sugar that you might as well just order anything at this point.