Dyno number variation is based off:
The gear you're in and how close to 1:1 it is
The weather
How the weather metrics are input
The dyno's calibration
Fuel quality
Airflow
The dyno operator
And most importantly dyno numbers on a closed dyno are irrelevant for comparison between car to car and day to day. They are only relevant in comparing to your own baseline.
360whp is a lot, and expecting 380 is around a 5% margin of error, so is it really that different? Are the other cars on other dyno really putting out 380 or are those fluffed numbers? At 380whp I expect the car to do the quarter in upper 11s/low 12s and near or over 120mph.
This video is a great tale of caution regarding comparing different cars on different dynos. Even dynos of the same brand can vary quite a bit. The only way to really judge your power improvements would be to make a baseline pull on this dyno before tuning, then you would have something to compare against.
Since this is a basic bolt on car, the owner could return to the dyno and test it on the factory calibration, the off the shelf map, and the custom tune all in the same session. Rudz is a top notch operation and I'd be confident that they can help you get the testing done in a such a way that he has great results to go by.
My biggest takeaway was that there was no baseline to compare the results to (but admittedly I stopped watching the video at the 10 min mark). For all we know, the car could have baselined at 280hp on the stock map. I'm no dyno expert either but I'm thinking it would have been wise to run the factory map a few times before the tune was loaded.