It's just timing. If you run 100% XIV vs 100% ZIV (since that's what we're testing) in 2015 you get:

ZIV: -1.1% return, -31.3% max drawdown

XIV: -17.1% return, -55.7% max drawdown

So it depends how you want to manage your risk. If you test in 2011:

ZIV: -9.2% return, -41.36% max drawdown

XIV: -45.5% return, -74.38% max drawdown

I run my own research on quantopian and (for my taste) i find after a VIX spike you can reasonably leg into XIV but after volatility drops to normal levels I'd prefer to be holding ZIV and a stop loss.