Because the wire bows under cutting resistance: the carriages reach the corner before the sagging middle of the wire does, so corners get rounded and curves trail. The faster the feed or the cooler the wire, the worse the lag.
Reduce it with more tension, a better heat/feed balance, and by slowing down on tight detail so the wire keeps up. Generous radii cut more faithfully than sharp inside corners. Some toolpaths add a brief dwell or lead-in at corners to let the wire catch up. If only the centre of a long span lags, that is classic bow — increase tension and tension-spring stiffness, and do not over-speed the feed.
Reduce it with more tension, a better heat/feed balance, and by slowing down on tight detail so the wire keeps up. Generous radii cut more faithfully than sharp inside corners. Some toolpaths add a brief dwell or lead-in at corners to let the wire catch up. If only the centre of a long span lags, that is classic bow — increase tension and tension-spring stiffness, and do not over-speed the feed.