Title says all I think :) When an asteroid is depleted, other miners come and mine it. Here's an screenshot of this in action: http://ovo.li/YEvyw8
Well, it's not a bug, it's a feature! :)
Just imagine that they have some different gear that can be used to extract some different ore, or maybe they are just mining for rocks? :)
Well played ;D
But now I want that device :P