Hi OnHoliday,
Randomness and Probability will take pitches from your sequence to trig notes randomly. This is meant to have a random sequence sounding in the good scale. If your sequence is composed of A3, D3, G3 notes, you will only hear A3, D3 and G3 notes in random mode. At the moment (current firmware), the drawback is: it will take pitches even from steps that are off (but only those before the last step). As default step value is C3, it will also trig C3 notes, even if there is no C3 note when your sequence is playing without randomness. If you transpose your sequence to D, it will trig D3 notes instead of C3. That is why you have notes from 3rd octave, whatever the transpose is.
If your sequence is C3, E3, G3, you will hear those notes, but with more C3 than E3 and G3 if there are steps that are not activated before the last step. A workaround would be to change the pitches of steps that are off also, in order to balance the randomness (eg: add more G3 and E3 on steps that are deactivated).
Hope this help,
Y.