The current core takes "as many pixels as possible" per line from the Amiga, even if they're way out of the overscan area. The same amount of pixels is pushed via DVI to the monitor, and that's over 1600 pixels per line for a PAL screen (where only 1536 are potentially valid). This is a known issue that you will be able to fix with the configtool.
Monitors with a smaller panel probably only display the "left part of the screen" and cut off whatever there is on the right. Some monitors scale, others don't, which is why you get different results with different monitors.
The vertical line on the very left of the screen should be outside the visible area. It's a debugging artefact - just ignore that :-)