Hi,
No idea. Makes sense: lower current permits higher temp, and vice versa. Maybe the calculator takes into account glued on copper layer separating from board at that current and temp., maybe it factors in when pcb may burn or set on fire, presumably it'll incorporate some safety standard which sets limitations on what you can safely do on x type of pcb, like minimum distance between AC mains and DC and so on.
Probably not for 40A, but most of the traces for high dissipation I've noticed show a puzzlingly narrow trace from the IC pin(s) which opens out into a much wider trace.
Have you soldered wires from copper plane to copper plane? It's messy, and unreliable, and a pita to get the wires to stick. With your mega taser vaporiser circuit there, and in case the solder may melt at hot points, I suggest using screw/round terminals, nuts, bolts and spring washers.