I’ll check with the main author, but my guess would be that the progression was meant to cover the standards which (as I suspect that you know) begin at K, not PK. Or is your question meant to be about why are there no PK standards?
PK territory is covered in the National Research Council report Mathematics Learning in Early Childhood: Paths Toward Excellence and Equity. This can be downloaded free of charge here: http://www.nap.edu/catalog.php?record_id=12519. In addition to discussion of research on early childhood mathematics, it’s got recommendations about the early childhood workforce and its professional development, and is accompanied by a podcast.