IRL, the shield is called tinbe and the spear, rochin.

In Rokugan, the reverse is true.

Why is that so? I've done some googling and found it appearing as the reverse in L5R since forever - is that a mistake that's carried over across publishers and editions, or is that intended for some reason?

Most likely someone flubed their research.

Dollars to donuts, it's because rondache is a shield.