When viewing my Lt Lesson's Revisions, why are there 'Published' (book) icons next to more than 1 different Revision of the Lesson?

It is possible to have two or more Revisions Published at the same time, if the same Lesson is both Published as a stand-alone Lesson and is part of a Module(s), or exists across multiple Modules.
Since Publishing changes to an individual Module or a stand-alone Lesson are done separately, it is possible for a stand-alone Lesson and different Modules to be operating under different Revisions of the same Lesson:

In the above example, the stand-alone ECG Basics Lesson has been updated to the latest Revision, whilst the Lesson as part of a Module has yet to be updated, and is running on a slightly older Revision. Since both Revisions are actually in use, a 'Published' icon is shown next to both.