Context: LowLevelHelpers.sol#L54
This rounds down the number of words (for example, if returndatasize() = 31 the correct number of words is 1). The number of words is defined rounded up in EVM.
For a precise calculation, it should be div(add(returndatasize(), 31), 32). A typical routine in internal compiler code. Here is how the Solidity compiler does it. 
The above issue can affect the gas computation for returndata, although unlikely to the point that it is severe as the computation is still making some assumptions about msize.
Replace the rounding from down to up.
