Posted: 15/09/2016 at 13:58
I'll do my best since nobody else has
From the flat-topped appearance I think these have already had their growing tips removed to stop upward growth and make the ball. From there they will have made multiple stems. Therefore no single stem to carry on upwards. The multiples will go on upwards. When you have removed a few lower branches you will find the base of these multiple stems rather than the single trunk.
I have no experience of this, just extrapolating from plant growth generally.