That may be the case sometimes, but I have one that has plenty of space for the 1st lower premolar that didn't come in.

It's there alright, on xray, it just did not break the gum line, the other did come in. This dog is not short muzzled, there was obviously room around the tooth, and she is in fact from multiple generations of high level hunt test and hunting dogs. Don't know which of the premolars were missing in the case of Diane's pups, but it seems like sometimes those first premolars are just too tiny to surface.

I do think you're right in the case of my short muzzled pet girl, who seems to be missing all four backmost molars, they may be there somewhere, like impacted wisdom teeth, or not there at all...