We're replacing a lavatory sink, and it appears that the bottom of the new sink is not flat. It rocks from side to side nearly 1/8 of an inch. It's a 19" round Gerber vitreous china sink. If we spin the sink 90 deg, it still rocks from side to side (relative to the sink, that is). Is this sort of thing normal? Or is the sink defective, and should we exchange it?