I'm not a medical expert or something, but via physics, if you blow inside a tube, you are basically applying a force on a given area which is one of the definitions of pressure. So pressure readings will increase. Also within a given volume, your breath is warm and contains moisture. This can also increase pressure as air molecules have some kinetic energy and collide on walls(Hence increasing force exerted thus pressure). also increasing volume e.g inhaling lungs causes pressure to decrease, thus the outside pressure is greater than the outside so air is forced inside the lungs(that's how breathing works).
So what you are seeing has got nothing to do with the sensor itself(the sensor is perfect, but its the scenario and environment that affects these readings.) Look up about pressure and gas laws. For what you are trying to make, measuring breathe pressure is very difficult because you need to take into account the volume of lungs, the flow rate of air, temperature etc. I don't know how they do it but it's not as simple as putting a sensor inside a tube and getting pressure readings because these values do not represent pressures inside the lungs. You could, however, use something inflatable which mimics lung volume, then place sensor inside it. that could work now I'm thinking about it. the temperature will still affect values which is fine because humans are naturally warm(so this is a constant). So if you actually want to see the pressure increase and decrease, increase and decrease the inflatable just like breathing. This was an overkill explanation but oh well
