Fed a font sop into a primitive sop, toggled on “Do Transformation” and scaled y to .2 – and the letters are centered/scaled at different heights - see image. The letters are polygons so they have basically the same bounds in Y, I don't get why this is happening…any ideas? I've never seen this before.

If I interpose a resample sop the centers change a little bit as well.
It's 10.0.595

Well it seems to me the primitive SOP is scaling around the centroid (aka barrycenter aka center of mass aka average of points) and not the center of the bounding box of the primitives. When you use a transform SOP you get the effect you expect. It's not something you would expect I think (but useful in some cases for sure). Nothing in the help primitive SOP help about it though.

It does make sense that when you add points the geometry shifts as it has more samples to calculate the average value.

You're right, now I know not to confuse centroid and center! Thanks for taking the time to set me straight.