Unfortunately the Y offset is equally applied at the bottom and the top of the taskbar. This has been suggested by me and others to make the top and bottom Y offsets separate settings. So far, this has not been implemented. I agree with you, it would be nice to have the gap at the bottom and not at the top.

The one thing you can do, is to turn off rounded corners on the taskbar when windows are maximized. Use the pull down shown below to adjust settings for maximized windows.