VMac,
Where are you positioning the probe in the barrel? I think that can make a big difference in accurate readings. Too close to the food or the barrel wall can lead to low readings in my experience. I like for it to be at least 2-3" away from the meat, and as close to the center of the barrel as possible. When the cooker is loaded down, this can be tough to do.
Next, is your stuff done when the videos say it should be? For example, if those three yard birds were done in 2 hours and your temp was reading 265 for the majority of the cook, I would guess that was a low reading and it was really running closer to 300.
On the other hand, if you crack the lid to get up around 300, and the chicken was done significantly sooner than 2 hours, then I would guess the true temp was 325 or even a little higher. I have had this scenario play out when the probe on my Maverick was too close to the birds. When I checked them at 2 hours they were way hotter than I thought they would be.
Just something else to consider.