Actually, and suprisingly, there is 1 Rocky Ice World and 1 High Metal Content World that made the list!
Yes I think that's the exception that demonstrates the rule. Rocky ice worlds may be a given depending on how much may be rock and how much may be ice. But HMC shouldn't be possible. The fact there is only one on the list is a good indication this may be a hand placed exception, much like Hi'iaka in Sol, a small icy body with a radius of 160km and a mass of .0003 earth but a gravity of 0.48g. Not just unlikely, but impossible according to our current understanding of physics. So a misplaced decimal point when the devs were placing these objects could certainly give us these strange anomalies. Hi'iaka obviously had a misplaced decimal point when they were setting the gravtiy, Epsilon Orionis AB 2 may have had a misplaced decimal point when they were setting density and when they were generated by the forge it gave anomalous results.
So I generally regard exceptions like this as just that, an exception. If there is one and it's possible according to the way stellar forge works, then there would be more because you never get one of anything in physics, if it's possible it will happen regularly, if it's not possible it will never happen.
The stellar forge is of course governed by rules, we may not know them all or understand exactly how it works, and of course it was written by human beings so a mistake or two creeping in may throw out the occasional outlier, but if it's working correctly we don't get exceptions, such as there are so far no procedurally generated bodies smaller than 137km radius.