It seems that both these terms get thrown around a lot. Both, I think, describe the same idea. Which was established first?
Also, it seems like some people describe it as a good thing to have a low x factor, while others describe it as a good thing to have a high x factor. Which is it?
Wikipedia says bus number is "more commonly known as truck number" But in the US, "hit by a bus" is practically an idiom, while "hit by a truck" is not (although either phrase is easily understood.) Regarding high/low being good, the wikipedia article says:
"High bus numbers are good (with the best case being equal to the number of developers on a project). This means all developers understand the codebase more or less equally. A low bus number (especially, one) represents a high risk."
I'd add to what @cartoonfox said: Promiscuous pair progamming is a good way to distribute critical knowledge around a team so that the truck number is as high as possible. If you don't swap pairs often and with many different team members, knowledge isn't distributed very quickly.