This is the kind of discussion I hoped would develop. Back when I was a young pro player the older guys I played with were often from the big band era and were very strict about rhythm and groove. Being able to swing while maintaining tempo was the goal. Maybe that's why it's something I've always been aware of. The groups I was referring to in my OP were mainly small, mostly pickup jazz outfits. I wasn't trying to make value judgements, simply noticing it on some of these recordings and speculating on the possible reasons. Of course there are many, but that's what makes it interesting IMO. I'm too lazy to list all the stuff I listened to and didn't write them down in any case, but anybody can do the same thing and see if they agree or not. It can also help to develop deeper listening skills, I believe, which we can all use.