They are for two different types of foundation elements.
ASABE EP486 is for shallow post footings - large hole, spread footing at the bottom of the hole, post resting on or attached to footing, and the hole backfilled and compacted around the post. Big signs, pole barns, etc.. Can handle large...
That is an older sketch with variable names that never got updated to match the current design guide. It's showing S as the post spacing and L as the post height and no gap at the bottom of the fencing.
For flexible fencing like chain link, if the post spacing is much smaller than the post...
IBC is not considering a fence post, just any post resisting any type of load and they happened to use h for the force application height to get the moment that must be resisted by the footing.
For a fence post, applying the force at the mid-height provides the equivalent moment at the base as...
IBC footing design is not worded very clearly. I've got about 9 pages explaining the IBC footing design proceedure at https://fencedesign.com/footing-design/
