There may be application specific reasons to set different defaults for individual spacing constraints, asking for a general answer doesn't make much sense however. If track to track spacing is marginal, you may want to mitigate those constraints that are not essential for design density. A trivial explanation could be that the DFM rules had been edited for a new design, but incompletely.
In my view, the question brings up a simple counter question. Do you understand the technology parameters behind the present design rules? If not, you should enquire about it.