Mutt, this is one of the greatest frustrations of smoking and is experienced by all smokers regardless of equipment or experience.
Meat plays a large role ... while buying from a butcher who sources exclusively from a specific farm or ranch would be ideal, most of us shop at big box retailers who receive their meats and poultry from a variety of producers. Feed and care of the animal will affect the meat, as will the handling both prior to slaughter and after, the skill of the butcher, the care of the packaged product while in the store ... the list goes on and on.
Then you throw in our prep methods. While a good process and an adherence to said process will be huge, if we have to make a new batch of rub between cuts, or the proximity to the smoke box or top vent, or even slight differences in the resting cycle can individually or cumulatively affect the quality of the finished barbecue.
More than likely, you're seeing a combo of meat quality and proximity to the heating element/smoker box. Next time, stick to a replicable process (rub prep, immediate refrigeration under ideal temps, and then a 3-2-1 method for spares or a 2-2-1 for babybacks and you'll be good to go.
If you're still wanting to do a little Scooby Doo work, perhaps take a single rack of ribs and cut it in half prior to your prep and then deduce whether placement in the smoker resulted in the outcome you observed. Nonetheless, congrats on a good meal to close out the weekend!
