I noticed that sometimes, as well. I assume the difference in gold would be due to rounding, since there are percentage based modifiers to country revenue but the strategy map only displays integers. If a country has a gold revenue of 3.6, then it may be rounded up on the strategy map to 4, but when doubled, rounded down to 7.
In a save in act 2 where I had taken over 75% of the countries, Bethune had a listed revenue of 6, which went to 12 with a gold mine. Eadwulf had a revenue of 4, which jumped to 8 (dwarves opinion 50%). In a later save I had been delaying ending the act so I could go though some options with a different wife, Bethune had a revenue of 8, which went to 15 with a gold mine. Eadwulf's revenue of 6 whent up to 11 (dwarven opinion 55% and with a 'Country gold revenue' adjustment of 20.4% listed in the Empire Overview). Steelside had a revenue of 10, which went up to 20, though (in the earlier save it had a revenue of 4, or 8 with a gold mine).
In either save countries with a revenue of 1, 2 or 3 doubled as expected.
