The default zip on Xero bags is a short zip with a gusset. This is a fail-safe design. The short zip is backed by a down-filled gusset, so that when the zip is opened, the bag expands instead of opening right up. Not good for ventilation, but extra security in extreme situations in case of zip failure:
A full-length zip protected by double draft tubes is available as an option. Full zip bags ventilate well, so they can be used over a wider range of temperatures:
Our full briefing on sleeping bag zips.