So coinbase has recently announced that they can no longer do free transactions as they are experiencing rapid growth and has introduced a fee for sending bitcoin since the miner fees have gotten higher. Some the fee can be as high as $3.00 worth of btc (as of this day June 27th 2017)

But how can they still do free transactions between bitcoin addresses on coinbase? Is it cheaper to send btc within the same wallet or something? Or does coinbase not actually send bitcoin but simply adjust balances internally within their website?