If you mean in generic 4:3 mode, it could be because the Horizontal Active area is 1536 in Lx5, which is an irregular number, and your display feels it needs to stretch this for some reason. For “regular” 16:9 & 4:3, H.Active would be 1920 & 1440, respectively.
The other reason is that in Lx5, the top and bottom is cut off, showing only 216 of the original 224/240 lines. Do note that this shouldn’t appear stretched really, more like it is zoomed in.
Edit: realised it could also be like my old TV, where I had to reduce samplerate. In that case, a samplerate around 1950 should be ideal. Then reduce H.backporch and (possibly) H.sync as needed.