Actually the number of lines per second is not constant, but the overall video BW is “similar”. In terms of analog TV, PAL has at the same time lower refresh rate and higher channel BW, which in terms of computer resolutions means that NTSC is exactly 640x480 (as that is derived from NTSC timing) while PAL is something like 800x600. Home computers for all the time where that class even existed simply didn’t have both the framebuffer sizes and video BW to generate full resolution video, so the resolutions ended up halved (and in analog TV there is no horizontal resolution to speak of, so 320 is just as good number as any)
Akshually, the horizontal (BW) resolution has little to do with timing. It's as good as the analog components are and as good the computer can output. Amigas had 1280x256 resolution, if you wanted, on standard PAL equipment. (On a fuzzy TV set, you weren't likely to appreciate 1280 horizontal resolution though.)
