Apparently they have chosen to configure the sequencer in that way so that if you want to transpose down from the lowest note programmable you won't go below the lowest MIDI note (or something along those lines). I've done the same thing for some of my bass parts to reach that lower 2 octaves; I transpose the sequence down before I record the sequence (using the BSP's pads) then save so I don't need to worry about the sequence need transposing every time I play it. Another way to do it is to transpose your synth (or down-tune the osc's) to the lower pitch you want, but the you need to remember to use that patch every time, or the case of an old analogue synth with no patch saving, retune/detune the synth.