So it's been... 6 months since the last post here, but here's why Super Mario Bros. Deluxe (GBC) can get 242 in 2-3, while the NES-NTSC version can only get 240. (There are a couple of things I just found in the past few days.)
1. In SMB Deluxe, Mario travels 3.5 fewer blocks of horizontal distance in 2-3. This seems to save 22 frames; 1 second on the game's timer equals 24 frames.
- The third platform is 2 blocks shorter in length than in NES/SNES, 10 blocks long instead of 12 blocks. (2 block advantage)
- The end staircase is missing one step, so that it's not too tall for the small screen. Thus, the staircase is also 1 block shorter in length. The flagpole is not shorter, but the jump feels the same because the jump to the flagpole is 1 block shorter in length. (2 block advantage)
- Mario starts out 0.5 blocks behind his NES/SNES starting position. (0.5 block disadvantage)
2. In SMB Deluxe, Mario's acceleration on the ground is faster than in NES-NTSC, saving 6 frames at the beginning.
These two things mean Deluxe saves 28 frames (1 game second plus 4 frames), but it actually saves only 25-27 frames relative to the game timer, due to timer technicalities. Still, it means slightly more than a game second is saved. Since NES-NTSC can get extremely close to 241 (maybe even 1 frame away), this means Deluxe can go all the way to 242.
Also, I think NES-PAL can get 241. It has the faster ground acceleration (point 2 above), but the level is the same as NES-NTSC.