ETF is inherently to track those index movement, but X3 leverage sometime can be very hard to track by this ETF. Their job is to track those movement but sometime they can miss by miles on that movement also. So they basically tell you they will try their best but not guarantee.

For example, last week the S&P Volatility Index(ViX) spike a whopping 70% in 5 days, over the same period its ETN (VXX) follow through by a 30% movement only.