From what I see it sends a pulse for 10 microSec and it measures the time that echoPin is in HIGH state. So why the result isn t always 10 microSec ?
I don't understand the logical process that got you to that conclusion. The trigger pulse causes the sensor to transmit a burst of ultrasound, and at the same time it raise the echo pin. When the sensor receives an echo of its transmitted burst, it lowers the echo pin.