ALTS are more volatile, also a reason why some traders who don't use leverage/margin, trade ALTS with and against BTC because of the larger percentage difference in Price Action.
ETC is one of those that have a wider range in that same Price Action.
BTC goes down a little, ETC goes down probably twice as much.
If somebody has time on their hands, they could drop the calculation, but that is basically it.
Also more to gain percentage wise with Alts like ETC.