Weird stuff happens in plastic molding. I highly doubt they are using a different mold for X Buzzzes, it just so happens that X plastic tends to give discs way bigger domes than Z plastic, which likely is related to the diameter issue. I just did a comparison of a recent run pop top X Nuke and a FR Z Nuke and the X has an appreciably smaller diameter too. A while ago I noticed my Yeti Aviars were larger in diameter than my KC Pros and DX P&As, despite being listed as the same on the PDGA specs. Likely due to the concavity and the plastic blend, not the mold being a wider diameter.
Different blends just mold up differently, and there's not much any company has been able to do about that yet.