got the answer from the ppls at subaru AUSTRALIA....

email as follows...

Thank you for visiting the Subaru web site and for your email.

To answer your question, WRX stands for World Rally Car X. The X
differentiates in the sense of "X opposed to Y".

Thank you for your email and I hope that this information helps put a
rest to any potential future arguments.

Kind Regards
Michelle Samuel
Customer Relations Representative
Subaru (Aust) Pty. Limited

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huh...?