If you're using the TWRP 2.7 build from its own thread on the Development forum, that's why. As stated several times in the thread, that one doesn't work, and the Dev never ever addressed it.
Use the TWRP from brothaedhungs Pacman thread in Development (not Original Development) and it'll work fine.
I've been using it since he put it up, and haven't needed to resort to using CWM since the early days of 4.3
Main: HTS One M8 stock rooted
Traded: LG Optimus G E970, 4.1.2
Loaned: HTC Inspire 4g, PACMAN
Auxiliary: HTC Incredible, tinyCM10.1
Auxiliary: Nook Color, MirageCM7