A triode operates within the linear region of its characteristics....

In the linear region of the characteristics of a triode, the plate current $I_p$ varies linearly with the plate voltage $V_p$ at a constant grid voltage $V_g$.

The dynamic plate resistance is defined as $r_p = \left( \dfrac{\Delta V_p}{\Delta I_p} \right)_{V_g = \text{constant}}$.

Since the graph of $I_p$ versus $V_p$ is a straight line in the linear region, its slope $\dfrac{\Delta I_p}{\Delta V_p}$ is constant.

Therefore, the dynamic plate resistance $r_p$ remains almost the same when the plate voltage is slightly increased.