I took this out of the GP32 Non-FLU vs FLU because I didn't think it was very on-topic. I'll try answering the topic here.
First, I'll give the short answer: No emulator is perfect. Outrun does something the emulator hasn't been coded to handle properly, yet.
Now, for the long answer:
I'm not a PCEngine hardware specialist, but from my experience, but the main reason proprietary systems are not emulated perfectly is because we had to (hopefulyl legally) reverse engineer their systems to figure out how they work. Outrun could be one of those games which does something unexpected, in a way the programmers cannot easily fix.
You'll see this many times in emulators, something like "if (AERO_ACROBAT) TILE_COUNT++; //hack for Aero AcroBat 1", where the real underlying truth is that they don't know exactly why that hack is necessary - obviously the SNES cannot preform such a feat, because Aero Acrobat was released after the SNES, and (supposedly) all the game data from Aero Acrobat is contained within the ROM emulated.
We especially have the problem with the PCEngine, because it's not as popular as SNES and the like, so there is less accurate source code available. GPEngine did it best with the programmers and existing source code which could be found.
In truth, we could have the Playstation One emulated on the GP32, but it would involve massive amounts of compression, decompression, possibly the 32MB upgrade to the GP32, and a team of dedicated programmers possibly working from the ground up in ASM. Also, in truth, I could gain magical powers to fight evil alien forces from the Orion Belt. When there is insufficient information to present a logical conclusion, the possibility still remains. I'm merely denoting this possibility exists, however unlikely, hoping to draw a parallel in your mind.
By now, you may be wondering 1) why I wrote so much, and 2) why you read so much (unless you skipped down here). I think it has become apparent to some people on this board that I am not one for brevity, in some cases. Additionally, I meant no personal offense to the original poster, Muzzaro, by making this so long-winded. I mearly wanted to clarify a situation which I have found other people confused on as well. Only read the first sentence in the explanation if you don't want to interalize the extra information.