Command arrow in NSMatrix

I am confused about the behavior of the arrow keys in NSMatrix. Just Arrow will select the next cell in the direction of the arrow. However Command-Arrow seems to have an inconsistent behavior. When there is a single column, Command-Arrow will behave the same, it just selects the next/previous cell. However, when there are more than one columns, it only changes the focussed cell, not the selection. I can imagine one or the other to happen, however shouldn't this be consistent, i.e. always behave the same? Is this a bug?

Christiaan