# Why does a turbine engine compressor ratio affect stagnation pressure?

This might just be a misunderstanding on my part, but in an ideal Brayton cycle, the flow through the compressor and turbine is isentropic, meaning $$\Delta s = 0$$. However, usually you are given the compressor pressure ratio to calculate the exit stagnation pressure, i.e. $$p_{cr} \equiv \frac{p_{03}}{p_{02}},$$ where station 3 is the exit and station 2 is the inlet. This makes completely no sense to me. In an isentropic process, the stagnation pressure is constant, so I don't understand how there can be a ratio. I do understand that the pressure increases in the compressor due to compressing the air and increasing the temperature, but these two points of view seem to be contradictory.