Your position isn't moved as far doing forward jumps opposed to backwards jumps. That's why we do it that way.
It's not an emulator problem, it's just a position and execution problem.
Here's a video of FCEUX I made from 1-1 doing forward jumps, got to maximum x value a little before the "stairs" - tried it on NES (AVS) and couldn't replicate.
Did it on AVS with the original cartrige, not the powerpak. The forward jump advancement is WAY bigger on emulator than on NES
Here's the video from the original post getting the wrong warp in 4-2 without any backwards jumps
Skip to 0:38 for the run without backwards jumps
Forward jumps still move you forward, just not as much. It looks like you got five bumps in that run, which is probably usually enough to make the wrong warp work.
I take a route on 4-2 that gives me four bump opportunities, since I often miss one. On the occasions where I hit two backwards jumps and two forward bumps, it almost always works even with a sloppy pipe entry (i.e. not leftmost pixel).
However, if you think the emulator is doing something differently than the real hardware, a first step might be to record your inputs and try them against a different emulator (e.g. Bizhawk).
There are lots of other ways FCEUX (at least the two versions I've played on my two computers) differs from NES hardware...
¤ Lakatu stays on the left side of the screen during 8-1 (if you're perfect pace), whereas on NES he doesn't ¤ While "running" in 1-2, you won't land on lip of the second pipe, unless you slow down, on emulator - on NES you usually do ¤ on 8-2, the intro strategy doesn't work with lakatu's dropped baddie - you always run into him (when jumping first on the second block) - but on NES it's fine. ¤ I still hold that forward jumps move your xpos more in FCEUX than it does in NES, but I haven't conclusively shown that - although I rarely don't get the WW on emu, and am still very bad at it on NES... that and look at the first video of 1-1, I tried that on NES (but didn't record) and didn't move as far forward.
It appears that there are some pretty clear differences between emu and NES, at least from what I've seen... and I'm sure that there are more, I haven't really "figured out" the speedrun past 8-2, so I don't know of any obvious discrepancies beyond that point. I'd do a "TAS" style input comparison, which would be a much better test, but I have no idea how to do that on the actual NES hardware.
Switched to NTSC rom and... it's all normal - I can't delete the thread - but, all of my concerns apparently only apply to the PAL rom. Case closed.
Found a YouTube video illustrating the difference (not mine)
Effective immediately, any submissions using the SNES9x emulator will not be verified. The SMB1 mod team has concerns about its integrity due to its ability to overclock and remove lag frames, which may not be easily detectable. Additionally, we request that all BizHawk submissions avoid using the S