It's a bit more complicated than that. LX (and many other airlines) distributes different availability to GDS' depending on the point of sale, i.e. the location of the travel agent. For example, a ZRH-based travel agent gets to see different availability for a specific flight through Sabre than a BOS-based agent does through the same system.

Swiss.com also shows different prices & availability depending on point of sale. It assumes your first origin as POS and also quotes in this currency. (Example: MAD-ZRH-MAD, priced from POS Spain, frequently has lower fare classes available than the same flights booked ZRH-MAD-ZRH with POS Switzerland.)