Paul Krugman's guess[1] is that since Putin wants to sell a smoke-and-mirrors idea of "Everything is fine, Russia is winning!", having a number (the exchange rate) not match his bullshit would mean an obvious "This guy is full of shit" to anyone paying attention. So, he's made the central bank boss[2] (a respected competent lady, who rumors say tried to quit but was not allowed by Putin) do anything she could to make it look decent.