I think the answer to the specific original question is that the orange lane marker squeezes the traffic into one lane because the road at that exit point is a one lane road.
Where they don't have the orange lane marker, you can exit from the left lane of the roundabout into the left lane of the road you are entering (e.g. Morse or Buena Vista).
However, for example, at the Morse-Bonita roundabout, Bonita has only one lane to accept traffic from the roundabout, hence the roundabout has only one lane at that exit point.
EDIT: the diagram doesn't display that, but the aerial photo posted in a subsequent post does illustrate it.
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