Although i do not know the answer to your question, I think it may be for two reasons. The job will look better and cleaner from the outside. The fused part between the clear and the solid fabric is hidden from UV damage.
If I were having a new Bimini made designed to incorporate clear and solid materials, I would insist the clear be attached from the inside. I like the solid material which is polypropylene coated on both sides because it can be heat welded to itself and/or clear material, eliminating the need for seams which will either eventually leak or deteriorate because of UV.
If you are having a new Bimini made and plan to have the seams heat welded, chose your contractor carefully. Before giving him an order have him give you an actual sample of his heat welding, including clear to solid, then hold him to the sample you approve.
Hope this helps.