nothing is "wrong",....batteries read 13.2 and 13.4 volts when fully charged, I tried them hooked in series and in parallel. both starters (one is brand spanking new) are clean and in good condition with clean unoxidized contact points on the switches. nothing about them is any different than any other starter out there, that I can tell I checked them for bearing alignment and both are as free as they can be, magnets feel strong and about the same, cord and clamps are good and battery terminals are clean....engine (Magnum .46XLS) runs perfect once started is free spinning and broken in properly, so there is no drastic tight spot at TDC. I've put both starters on the battery in my Jeep and as you say, it makes no difference because of the batteries cranking capacity. (still very easily held from spinning with my hand from a dead start or when gripped while spinning) I have even replaced the clamps with terminals and bolted the terminals to the battery lugs.
I've been around the block a few times with all sorts of machinery in my 63 years,....believe me,....there isn't anything you can tell me that I don't know or haven't tried, to make these starters perform the way they should...... evidently,.... both of mine are just garbage..