it drifts left on the floor because it is "absolutely level" like if the heli had a gyroscope level gauge or bubble indicator.

the heli when it's in the air will tilt to the right a little bit in hover. that's why you cannot move right on the floor because it cannot tilt to the right. adding right stick will tilt it further to move it easily in air.

basically do NOT try to keep the heli level in hover. you should be focusing on keeping the UNIT still to hover. you only think about absolutely level when you are landing - and even then you dont. you only think about level when it comes to elevator so you dont hit the tail on the floor. landing 1 skid at a time is how it usually happens anyway, just dont let it tip over sideways.

if you do this on the floor, the natural position of the heli will be up on the left skid and light on the right (but not moving) if you give it enough pitch/throtle. that will be True hover on the floor. you find you will be able to move right and left the same.

your heli is drifting to the left for the following reasons:

the tail rotor is adding left vector thrust
the heli is unable to shift its weight to the right to compensate for the tail rotor by tilting right
the trim is NOT off, the heli just cannot be controlled as perfectly on the floor versus in the air
you do not have 100% control of the heli until it is given enough power to lift off.