Question: Order in plots[display]

with(plots):with(plottools):
p1:=plot( x^3,x=-1..1,thickness=20,color=red):
p2:=plot(-x^3,x=-1..1,thickness=20,color=blue):
p3:=display(rectangle([0.5, 1],[0.75,-1],color=green)):
p4:=display(rectangle([-1,-0.1],[1,0.1],color=yellow)):

display(p1,p2,p3,p4); # order = 4312

 

#  display(p3,p4,p1,p2); # order = 4312 (the same)

It seems that the rectangles are plotted first, in reversed order, and then the curves, in direct order.
Has someone an explanation?

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