While I don't own a PHEV, I suspect that when you run down the battery and it switches you to hybrid mode, the battery charge indicator will not change much (it will not attempt to fully recharge the battery). In other words, it will behave just like a hybrid (and my hybrid) and under most conditions the battery charge will remain in a narrow range. If it switches at 20%, it will stay centered around 20%.
PHEV's larger battery have an advantage in mountainous terrain. Larger capacity means they can recover more energy on a downhill (depending on charge status) than the smaller hybrid battery. And it is mountainous terrain that causes the largest swing on my hybrid battery, discharging deeper as it adds torque to the engine output on uphills, and charging higher than usual during long downhills.