1970 Dart matching numbers 340 4 Speed

I don't know. There seems to be quite a variety of circuits that got run through grommets depending on the year. 1967 had the tachometer wire added to the main harness and used the bulkhead connector. 1968 and '69 ran the tach wire through a grommet. There's many more examples like your talking about in the mid '70s. In this post Keith was comparing '74 to '75.

I'm going to guess the decision on which wires to route through a grommet came during production engineering. Production would project which options would be the least volume and easiest to add by using a grommet. They might have also decided grommet routing would be easier vs. having an unused wire occupying the standard harness bundle.

Exception to this might be items that the design team specifically called out as preferable to use a grommet because of the wire size or high loads. But even there, those were less common options...