The 2015 was the first table to address deck spans and cantilevers. It's not easy to do. We revised it in the 2018, but it still had confusing parts. We revised it again in the 2021 it is by far the best yet. I encourage everyone to use the 2021 IRC Section 507 Deck provisions as an alternative.
Now, to answer your question. The 2015 max joist span table is split in two. One side is for joists "with no cantilever" and the other for "joists with cantilevers". In order to make it work out every time that the cantilever could be 1/4 the backspan, the span has to be limited. In some conditions, when the span gets near the max, the cantilever maximum begins to shrink again. This is the problem that has caused us to revise the table each edition.
The footnote about the cantilever being the up to the depth of the joist is saying that with a cantilever that small you can still use the side of the span table for "joists without cantilever".
Hopefully that makes sense, but I hope even more that people use the 2021 version of this table.
Now, if you are curious about what is confusing in this table in the 2018 IRC and how we fixed it in the 2021, well I happen to have that exact video from my On-Demand Courses as the sample video at
www.BuildingCodeCollege.com The cantilever discussion starts at about 14 minutes. I can dig up my old video about the 2015 change to the 2018 if I'm asked.
I'm happy to answer any follow up questions. I've also got 12 hours of On-Demand deck code courses from the 2021 IRC available online.
www.DeckCodes.com