You guys can back me up on this - but I'll take a stab at it.
MTC is more negative during startup because the Boric Acid Concentration will be higher during startup than during power operation at rated conditions. As power is increased towards rated thermal power, Power Defect will add negative reactivity, which will be countered by dilution.
I'm a BWR guy, so that's as good as I can do over there in PWR land.