I am thinking it is part of a larger thing (although not sure it effects any game play). I am thinking that this is related to the bank deposit hiccup and the listing of city bonuses. I think when you first enter a city, defenses show what they would be without minimums, no city bonuses, and runs the bank at the 20% deposit rate. If you refresh the page, the defenses jump to the 10000 minimum (if they aren't past that), city bonuses appear including any bank deposit bonuses, and the bank then operates on the adjusted deposit rate of 18% or whatever that city is at. As the bank deposit is the only thing you do directly on that page, not sure it really hurts anything unless it somehow doesn't apply dueling type city bonuses if you are attacked in that moment.