I have a similar setup with the same kind of sensor. It just doesnt register the matress at all. My guess is that the matress weight is evenly distributed so it doesnt actually put that much pressure on the sensor.
water is 1kg/dm3, and acording to this website (https://www.aqua-calc.com/page/density-table/substance/human...) human is arround 1000kg/m3 which make sense.
So, yes matress is light compared to human, I guess it does nothing to the sensor pad.