Help understanding Excel's PercentRank.Inc vs .Exc

In the olden days, Excel had just one PercentRank function. It now has 2: PercentRank.Inc (replaces PercentRank) and PercentRank.Exc.

I understand how they work. I do not understand why they work the way they do and why I would use one vs the other.

This MVP says that the old function and the new .Inc function use a "slightly less accurate algorithm".

Can someone explain to me(a) why I would use one vs the other and (b) why the .Inc algorithm is less accurate?