Different pellets have different burn characteristics depending on their composition. Alder is a light, fast burning hardwood that gives little smoke flavor but high BTU's. Same with oak but to a lesser degree. If you put your 3 speed controller on ony one speed changing pellets can change temps by quite a bit since the auger on/off ratios and fan speeds stay the same. Even some of the Traeger digital contollers still function in that manner. Controllers, such a the Green Mountain Grill, adjust fan speed and pellet feed to maintain the set temp, so in those models changing pellets will not change grate temps. but it will change the pellet consumption rate to compensate for the difference in BTU's put out by the pellet. This is one reason some type of pellets will not get a pellet grill up to the advertised high end heat setting. There is also a difference in pellet density, with Traeger pellet being on the "soft" side IMO. That mey give them a boost in BTU output. Pellet size swon't matter because long or short they take up the same amount of room once in the auger.
Although all the pellets are technically "hardwoods", each type of hardwood has a different density, burn rate and BTU output, plus the pellet making process adds pellet density to the equation.
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