My belief is the two biggest factors you will see when buying a bag of pellets are:
A) Moisture content. This will vary buy several percentage points and since you are buying by weight, the higher water content, the less wood you will get per bag. Also, chances are the higher moisture content will also lower your burn temperature or increase the amount of pellets you will use.
B) Ash content. Ash doesn't burn. The higher non-burnable product you have (non-flammable impurities) the more non-burnable wood you will have in a bag of pellets.
Some math:
Gas grills can range from 35,000 BTU per hour to 50,000+.
For ease of math, we'll say we are running our pellet grills for an hour at a temperature that makes us use 24,000 BTU per hour.
To get the 24,000 per hour you would need the following amount of flammable wood to get 24,000 BTU in that hour.
A 6,000 BTU wood = 4 lbs
A 7,000 BTU wood = 3.5 lbs
A 8,000 BTU wood = 3 lbs of flammable wood burned.
If you 8,000 BTU wood was at 15% ash and moisture (total combined), you're using close to 3.5 lbs of pellets.
Also, if your moisture percentage will be 8% or less, so you have less water in your burn which would affect temperature differences between two similar woods.
It's unknown what an additional 7% of water / ash will do to a fire, but my assumption is that it would burn cooler / use more pellets.
PFI certifies pellet fuels on moisture and ash where bbq pellets are unregulated so you don't really know what you are getting since they aren't tested.
From using both, I've seen from my own experience the massive reduction in ash and less pellet usage from burning PFI certified pellets (not to mention the much lower cost).
If I want a specific smoke flavor, I use a smoke tube with a few ounces of bbq pellets rather than burn pounds and pounds of them as a heat source.