It is not by design. Stop posting nonsense.
I had two different BCBs, and one sagged on one side like that, and another sat perfectly flush.
There is a lot of variance amongst Apple's bands. Some sport bands wiggle and move around, some don't. Some BCBs have that sag, some don't. And I've seen link bracelets and milanese loops with their own variances (gaps at the connection point on some, not on others).
It is certainly NOT by design.
If it bothers anyone, you can try swapping it out. But you may end up playing a long game of swapping.
It's a first gen product, and quality control/machine specs seem to be all over the place.