Like with most implementation decisions, you can achieve the same result in many different ways.
Maybe they based the TC on the qlvls of the items, or something like that. Or maybe they just wanted a better correlation between the TC number and mlvls. Or maybe they didn't make concious decision at all and just implemented it one way, because an implementation had to be done, but the details might not have been specified (as this isn't visible anywhere I don't think the game designers even thought about specifying it).