Clojurians Log v2

I'm looking at the code for APersistentVector and there is a piece of code that I am having trouble understanding. I'm comparing doEquals with doEquiv. I would expect them to be quite similar, the primary difference being that s as you move through the vector and thing you're comparing against one will use Util.equal to compare items and the other will use Util.equiv. Each has three cases: (1) comparing against an IPersistentVector; (2) comparing against a java.util.List; (3) otherwise. For cases (1) and (3), indeed they are identical except for Util.equal vs Util.equiv. But for case (2), there is a difference. In doEquals, there is a short-circuit test to get us out early:

Collection ma = (Collection) obj;     // obj is what are comparing to, v is IPersistentVector 		
if(ma.size() != v.count() || ma.hashCode() != v.hashCode())
    return false;

In doEquiv, the short-circuit code is

Collection ma = (Collection) obj; 		
if((!(ma instanceof IPersistentCollection) || (ma instanceof Counted)) && (ma.size() != v.count()))
    return false;

I can understand the hashcode check going away -- more things are 'equiv' than 'equals' and the hashcode consistency will only work for Equals. I read the guard in the doEquiv code as trying to avoid calling .size() on an IPersistentCollection that is not Counted so that we avoid a potentially costly computation. My question is: why is that guard not on the doEquals case? What am I missing? (Or is it just missing?)