Without that information we wouldn't know the relative size of the rim as compared to the main body.
Making an assumption that the only interpretation is that they misunderstood pi or were even mistaken in the measurements they took in light of the ability to make this vessel in the first place would seem the least likely.
I make neither assumption. I assume they were only giving approximate measurements. Biblical literalists can't make that assumption, however, so they have to rationalize it.