HeffsF150

I ordered A flowmaster double wall exhaust tip. When I got it I noticed the construction of the tip where you bolt it up to the exhaust is seperated from the actual tip but joins together at the end of the tip. If that makes sense. Its 2.5" to 3.5" but how the it's made it is 2.5 all the way through with just a bigger pipe surrounding it to look like a larger tip...

My question is why is this and is it for sound purposes? I've seen other dual wall tips and they were never like this?