if you want it done right, do it yourself. get some In House analytics
if you’re planting from seed, you’ll get all kinds of variation, so testing a dozen samples from around the field is advised. Presumably you’ve noticed the variation in your fields if you’ve done this a couple or three times already.
the lab is extracting your biomass to test it. extracting more biomass will give you a better average. extracting more samples will also help.
in a pinch you could perform a pilot extraction on a dozen plants chosen at random and have the lab analyse that, but doing the analysis yourself allows you to get harvest day right rather than just close enough.