A lot of defending range charts suggest to defend with range that is slightly tighter, then the UTG range

My question is "why?"

I tried out in Equilab and saw that the BU opening range has 40% equity vs UTG open range, and if the UTG raises 3BB we get pot odds 3/7.5 which is exactly 40% to break even. So, why are ranges tighter to defend and not wider to the contrary?

Thanks for the answer in advance