Measurements on ultrathin body negative-capacitance (NC) field-effect transistors are shown to display subthreshold behaviors that cannot be explained as a classical MOSFET. Subthreshold swing (SS) at low drain bias decreases with increased gate bias for devices measured over multiple gate lengths down to 30 nm. In addition, improvement in the SS relative to control devices shows a nonmonotonic dependence on the gate length. Using a Landau-Khalatnikov ferroelectric (FE) model calibrated with measured Capacitance-Voltage and combining it with TCAD simulations, we show that these anomalous behaviors can be quantitatively explained and interpreted as field-induced permittivity enhancement. The model predicts substantial scaling improvement at the end of the roadmap.