Airflow pattern has already been mentioned, and that certainly plays a large part in the potential for flavoring agents from the smoke to come into contact with the meat. Something else to consider is how aggressive the fan is and how complete the combustion is of the pellets in the burn pot. As I understand (could be wrong) most pellet grills have an aggressive fan to 100% fully ignite those pellets as quickly as possible. There are other pellet grills which are outliers (again, as I understand) who utilize a much less aggressive fan, to allow for the pellets to smolder for a short period of time before fully igniting. The smoldering pellets will produce more smoke (not necessarily more flavorful smoke) and for some people any smoke is good flavor.
This may be a bit controversial, but I fully believe that pellet grills lack smoke flavor because pellets are completely dry. When you run an offset smoker the wood is still between 15-20% water by weight, so when you start burning up those wood splits you're getting an absolutely insane amount of moisture which is forced through the cook chamber. In my cabinet smoker I use charcoal and wood chunks, but I also have a water pan directly over the hot coals which boils off some water. I'll do 100+Lbs of meat and burn off roughly 12Lbs of water in the water pan during a 10 hour cook.
Meat science tells us that smoke doesn't penetrate the meat and instead the smoke is a surface reaction. It also tells us that smoke sticks to things which are wet, so the more moisture you get through the cook chamber (and onto the meat) the more smoke flavor you'll get on your meat. If the pellets are almost completely devoid of water it's easy to figure out why pellets give off less flavor than wood burning cookers.
A YouTuber named Mad Scientist BBQ did an interesting video about the process of wood burning and converting into flavoring molecules. He also did a theoretical calculation of the amount of water going through an offset smoker due to wood combusting...I can't recall the exact amount but it was shockingly high. He also did a test with pellets where he baked out all the moisture from them and I think they had about 5% by weight...compare that to roughly 15-20% you'll see in moderately seasoned wood.
Here's the videos if you're interested.
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Why Do Pellet Smokers Produce Less Flavor Than Offsets?"
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Do Water Pans Improve BBQ?"