2/3 doesn't work with multiple monitors

I'm using two monitors, the left is 1024x768, the right is 1600x1200. 2/3 doesn't work, and I tried the following cases:

1. open 4 windows on the main monitor
2. open 4 windows on the other monitor
3. open 4 windows, 2 on each monitor
4. open many windows, 4 on the main window

When I press the 2/3 hotkey, some windows get resized and repositioned. It looks like winwrangler is using the whole screen area (2624x1200), but even then it doesn't work, the vertical window borders don't match.