What is more accurate about this emulation compared to others?
On accuracy in general:
Often, emulators have accuracy issues that aren't apparent with the vast majority of games. Sometimes, though, this becomes apparent through glitches not present in the original ("The Great GBA DMA Disaster", as described by Endrift, comes to mind), crashes that
don't happen with the original but result in buggy behavior, and some more obscure games just don't work or have major issues. Even some more popular games have some serious issues with accuracy, like the SNES game Tetris Attack's 2P mode which doesn't display properly on most emulators including SNES9X.
Often, sound accuracy also affects emulation; very few sound chips can actually be emulated perfectly (to name two better known ones, NES and more recently SNES). Sound inaccuracies result in things not quite sounding the way they should be, and in some extreme cases it can sound just plain bad (Genesis being a perfect example).
In most accuracy-focused emulators' cases, you can't just talk about one specific thing that makes it more accurate, rather, many. Emulators don't just have one thing to do to make it more accurate as there's always usually thousands of issues that are caused by lesser accuracy. Overall, this emulator has less bugs than numerous other games. Take a look at
these three pages for just a few of them.
EDIT: Also, check out this video:
. It doesn't demonstrate mGBA at all (and the GB(C) emulation isn't great considering the early state) but it still describes accuracy well.