Knitting Needles

I recently won an auction at Herrschners for a set of Bamboo knitting needles. I received them yesterday (great customer service!) and was looking at the needle conversion chart. The size 6 needles were 4.25mm.

All of my other size 6 needles are 4.00mm.

Why is there no standardization of knitting needles in the United States?

I guess I'll need to learn the metric system after all!