Dreamcast uses DTV timings (H=858, V=525 for 60Hz and H=864, V=625 for 50Hz), which is why sampling should utilize the same parameters for best quality. However, framebuffer size in DC games is 640×480 as far as I know. With CRT, top and bottom portion of the image can actually get lost in overscan region, so a 640×480 screen with a 1:1 PAR (pixel aspect ratio) would look a bit squished on a 4:3 CRT display as well (only around 640×448 is actually shown). With OSSC and no overscan, a modern TV displays 720×480 on 4:3 area, again with squished image if DC uses square PAR.
I wonder which PAR DC 240p test suite uses, and if it also looks a bit squished on a CRT as I suspect.