Hmmm, I had the same 25/30 experience. The 30 was my first time around as the Jullii on med/med. The 25 was on my second Roman campaign as the Scipii on hard/hard. Besides the difficulty differences, I noticed that I had taken 5 wonders in the Scipii campaign; maybe they count towards taking a province (I didn't manage to pick up any wonders by the time I hit 30 regions as the Jullii).
Don't forget the game doesn't count your starting regions in the 'regions captured' scroll, this could go someway to explaining it...