Why is there a difference between eth.estimateGas and the real cost in testRPC?

Question

When I want to deploy a smart contract, I use the eth.estimateGas before deploying my smart contract. This gave me 688180 Gas.

let bytecode = compiledContract.contracts['Document'].bytecode;
let gasEstimate = web3.eth.estimateGas({
    data: bytecode
});

But somehow this estimation is false, and I have to set manually the gas to some value e.g. 4700000.

var myContractReturned = MyContract.new(param1, param2, {
   from:mySenderAddress,
   data:bytecode,
   gas:4700000 // gasEstimate not working here}, 
function(err, myContract){....}

Using web3.eth.getTransactionReceipt, I can see that the real cost was 817540 Gas. I know that the gas that is not used is automatically refunded, so there is no real problem but I'm curious to know why such a difference!

Thanks in advance!