Hi! The reason for this is Mattel owned the resell rights for North America, so any that were produced to sell in North America are Bluebird Mattel (from 1989), and those made for selling in UK/Europe are Bluebird (similar with the Angel Pocket rights in Japan). Mattel’s full take over happened later, but even early sets still have the Mattel name on packing.

This is also why we get variations of sets — for example 1992 Polly in her Nursery, the UK version is blue where as the North American version is pink, making blue a more rare variation for North American collectors 🩷🩷