It could be the type of pellets you are using. Just heard from a grill owner in NC who was having temp issues. He switched back to BBQ'rs Delight and the grill came up to temp. So if your thermocouple is clean and the vents aren't blocked, then more likely its the pellets. We've seen this so many times over the years. Once pellets get manufactured and bagged, how they are stored either at the producer, distributor, retailer or consumer, if they are exposed to the elements and moisture, it WILL have an impact on how hot they burn.
And, what MAK Daddy said about your meat acting as a heat-sink. That is true as well. Just a natural by-product of thermodynamics and science.