Why is VIX index up nearly 20% vs. VXX ETF only up around 10%?

I know the VXX is correlated with the VIX index, but definitely don't move 100% in tandem, but why such a huge spread/gap in percentage terms when volatility hits?
Based on a TOS chart this morning, VIX index is up currently over 20%, while the VXX ETF is slightly above 10% only. However, I did notice the /VX index futures is a lot closer to the VXX ETF on a percentage basis.

 

Great point, it's just how it's calculated etc, eg VIX vs VXX UVXY VIXY etc, contango, contract #s

 

contango yo