[Chu Gai]

Over at AV123, there was a forum post (http://forums.av123.com/
showthread.php?t=41989) where an owner of Rocket speakers observed
that some his drivers were exhibiting an imprint from where I guess
the motor is attached to the aluminum cone. Other owners in that
thread have also mentioned that they've seen it too. My question is
why is this happening? A person posted that it's because the aluminum
is very light but if that was the case I'd expect a flood of reports
by Rocket owners experiencing the same thing. Could it be due to poor
quality control by the driver manufacturer (Peerless India I think)
with respect the aluminum thickness or variations in the alloy?
Anybody seen this and can offer some technical insight into this?