Hi Martin,

Despite an extensive search, I have been unable to locate any details regarding the mattress you refer to.

Having said that, logic leads me to think that the zip is there to allow the manufacturer to make the mattress separately to the topper and then insert the mattress into the cover which already has the topper attached. I trust that this helps.

It could also be that if the topper begins to fail in the future, the manufacturer may sell replacement covers with a new topper attached, to give the mattress a longer lease of life.

Kind Regards Gary