The prices I'm seeing from probikekit and wiggle look similar.

https://www.probikekit.com/bicycle-w.../11349819.html $356.49

wiggle.com | Campagnolo Zonda C17 Wheelset | Road Race Wheels $353.49 (is Shimano out of stock?)

I am seeing a disc brake version for a little more (out of stock?)
wiggle.com | Campagnolo Zonda C17 Disc Brake Wheelset (Bolt Thru 6 Bolt) | Road Race Wheels $653

Perhaps you're looking at a disc brake version.

Odd, Ribble is generally pretty consistent, but they do seem to be higher for the non disc version, but significantly lower for the disc version.
Campagnolo Zonda C17 Wheelset - Wheels - Ribble Cycles $454.77

Campagnolo Zonda Disc Brake AFS Wheelset - Wheels - Ribble Cycles $463.65

Perhaps the MSRP makes it all more complicated, with them offering a "discount" from the MRSP, as well as the disc//non-disc versions with different prices.

Beginning of the season sales??