Not sure - the rpt version has a lot more dependencies on things like libavcodec
I imagine it's modified for acceleration, however it also misses out transcoding acceleration that's available by using omx-player. There are instructions elsewhere on the net to recompile tvheadend to enable this on RPi, which allows a live, in-the-browser, transcoded stream direct in the tvHeadend interface (no VLC etc. required).
I suspect that the answer is that they've changed a lot, but it's still not an optimal package but there is bog-all information out there about what is changed and why. I tried to look through and it's hard to pin down exactly what they've done without literally downloading the entire source package and comparing it to the 4.2.4 official repo.
It would be really nice if we could just use the upstream tvHeadend packages or, failing that, have a proper changelog/SCM for the tvHeadend changes.
Also, the -rpt version seems to use a lot of unusual paths for no real reason - every time I compile tvHeadend myself, it chooses entirely different paths, so I doubt that an official tvvHeadend package can be a direct upgrade to the -rpt versions (especially given things like picon locations etc. changing).
Would be nice if whoever manages that package could a) document the changes, b) point to a repository that they coded it from, c) keep it vaguely up to date (it is potentially an Internet-facing web service for some people, after all, and a lot of HTTP digest authentication fixes have gone in since 4.2.4) or d) hand it up so that we can move to the official tvHeadend packages.