the tpgs are a rigged market. you will never get consistency and so you just cannot compare lcgs and ngc or pcgs. sometimes one will be more accurate or in line with what you would expect than another. the cynic in me would observe the bigger players in the market almost certainly get more favourable outcomes with slabbing.
i've sent coins in for cross-overs between all of them (without disclosing it to the tpgs in question) and you get all sorts of weird and wonderful outcomes.
grading really should be quite straightforward when you have a population of say 20 of the same coin and you need to rank them in order based on your own criteria!