This is by far the most common “issue” we see described on forums and facebook since we started in this hobby. It’s common to see new pilots post videos of their quads armed with no props on, and 2 motors spin up slowly faster than the rest, which stay at roughly idle rpm. Here’s a video describing this:
Firstly, don’t worry; this isn’t a problem with your quad at all. In fact it’s just the PID doing its job. So feel free to go outside and maiden your quad.
Your copter works using a PID controller. The job of the PID controller is to make the quad respond exactly to your stick inputs. What’s happening when the 2 motor spin up is the gyro is detecting that your quad is very slightly off centre in one axis, when it detects this it spins up 2 motors faster than the other 2 to correct this error. As your props are off, the error is correct, as the motors can’t provide/change thrust. Thus the error remains the same. Then the “I” term of the PID controller kicks it, this causes the PID to increase the speed of the two motors, (for the maths geeks out there, “I” stands for Integral, in this case the error is integrated and then fed to the PID controller, as the error is roughly constant, the motor speed increases linearly). This repeats itself continually and hence the motor speeds up!