Why is the TX power limited to 18 C6X?

I’ve made another post regarding this and havent received an answer. I also asked about the arbitrary limits that are put on the AP for TX power being 14 and was told that its due to EIRP limits. I can almost understand that, however several of the settings are wrong if the true answer is EIRP so im not totally buying it. However, with the C6X being stuck at 18 with the built in antenna, is not even close to an EIRP limit. SO… why will it not go passed 18 or do i have some setting not properly done?