If its not a simple HK/China difference...I would simply come down on "batch difference"....thousands of these ponies were made, with multiple different colour paint batches and probably in more than one factory...and blue/purple eye ponies are notorious to having variations in both colours [Cotton Candy, Moondancer, etc etc] plus the easily leaching red pigment seems to change purples to blues a lot...
Its always cool to have more than one of the same pony which such minor differences! Congrats