I was curious about a fact given in my biology textbook as to why the Japanese have 3 pairs o floating ribs unlike the rest who have just two?
Because your textbook is probably worded poorly.
It appears that the Japanese (and several other ethnic groups) apparently have a higher proclivity for the 10th rib to be a true floating rib, but every ethnic group has the chance of having a floating 10th rib.
With that said, the one McGraw-Hill book cited in the Wiki article is the only source I've seen for this higher morphology rate. I, personally, wouldn't trust just one source, and (if true) the rate may not be much higher.