The difference is at 0' there is 14.7 psia and at 6000' there is 11.8 psia. That is a difference of 2.9 psia. But the deal is if it was sealed up at 0 feet sea level then at 6000 feet altitude it would be 3 PSIA pressure inside it. I agree something is not correct here. The housing / dome should be able to at least handle twice that. Once you get enough pressure inside the main housing seal usually leaks.