Created attachment 4798 
current behaviour screenshot

When tiling windows the window on the side of to the panel will share its half of the screen with the panel, while the one on the other side will have half the screen. 
See attached screenshot (screen has 1920 pixels, 190 are used for the panel, 770 for the first window, 960 for the second
I would think the expected behaviour would be that the tiling would half the avaliable space after discounting the panel => (1920-190)/2 = 865 for each tiled window

In any case, thanks for creating and maintaining xfce!