If the blade has a smile and you're going straight down, then you'll only see swarf in the middle even on a perfectly flat hone. I'm not sure how significant the swarf patterns on the norton are because I get much more distinct swarf lines on the shaptons than the norton, so the same thing could be happening on the norton but the swarf is washed around and spread out more so it looks more even than it really is.

If you want to lap the shapton I'd use something finer than 600 grit, like maybe 2k grit sandpaper. Or if you have one of those synthetic marble tiles then just wet the stone and rub it directly on the tile - Shaptons are very soft stones and will quickly raise a slurry that speeds up the lapping.