I'll try to find the original conversion thread to see whether it was deliberate or an mere miscalculation.
After a bit of searching, I found where the Spider-Horse Conversion started.
The original Spider-Horse Rough Draft has the correct +0 melee for the Full Attack hooves, while the "completed Homebrew" in the Creature Catalog new 3.5 conversions thread has it two points too low.
So it looks like a simple transcription error snuck in while it was being copied over.
I might as well amend the version in my CC files for the next update - whenever that happens!