Why beamsize has discontinuity on element edge

When crossing an inclined element edge, beam can show discontinuity like if its size changes instantly at the edge. Like this

It's because of how beamsize is measured. Beamsize is measured perpendicular to its axis, not along the element edge. So while the beam is obviously continuous on the edge, its projections onto the axis' perpendicular have different sizes before and after the edge.