Ok I'm assuming only BTC/XXX markets are used for volumes as well.
I know it's more work, but they could use other markets (for example FRC/CNY) to calculate volumes even if they aren't taken into account for prices.
They just have to add all volumes in FRC (or whatever currency you're calculating) and then multiply for their usual price in usd to obtain a more accurate total volume in usd.

Anyway, the volume column is a nice addition.