The calculator is subtracting 4.5 fluid ounces per ounce of hops to account for absorption.
My little amount of research varies. Some state that leaf has greater absorption, some feel the difference b/w pellet and leaf is negligible, depending on how well the brewer is able to drain the kettle. The most efficient being squeezing a hop bag.
Unless you are using large amounts of leaf, I wouldn't be overly concerned. Your methods will determine the actual hop absorption losses.
Perhaps adding a few ounces of extra water per ounce of leaf would be prudent, but I don't strive for that kind of precision with my brewing.
Ymmv