I have a factory made ECE scrub with a perfectly round arc to the blade. 
And, I have an old beat up scrub that may have started life as a smoother 
or something else, but the point is that the arc on that one is nowhere 
near perfect.  Actually it has a bit less of an arc than the ECE plane, 
and that can come in handy.  Basically I have a "rough scrub" and a "fine 
scrub" plane, when I don't want to hog as much wood off I use the fine 
scrub. 

Generally you are using a scrub for quick removal of wood and you will 
follow with other planes to get the final surface, so the exactness of the 
arc is not important, just so it is mostly even.  At least in my humble 
opinion.

Hunter in Richmond