I think the phase "Some Light cavalry" is only ever used in the Scenario special rules
e.g.
All Seljuq Turk Medium and Heavy cavalry units are armed with bows. No need to place bow markers.
Some Light Cavalry are armed with bows. Use the proper bow and non-bow Light Cavalry blocks for these different units.
This is to highlight the distinction between LC (Light cavalry) and Light Bow cavalry (LBC) which is designated in the scenario setup map
So for example, in the 4th hexrow down, the Turk units are marked as either LC or LBC, depending on whether they have bows.Different unit blocks and graphic, so it seems overly pedantic to me, but I suppose it was a confusion at some point.