Unsure if LEED form is behaving correctly

My project is pursuing Option 1, stormwater assessment report, of the Full documentation option of the O&M submittal path (what a mouthful!) of SSc6. The site was designed to infiltrate the stormwater volumes created by a 100 year storm, so has the capacity to infiltrate all volumes created by the 2 year storm. I'm entering the same volumes into the "Two year, 24 hour design storm onsite rainfall volume" and the "Mitigated design storm rainfall volume." When I do this the calculator (Both on the downloadable form and the LEEDonline version) says that the site mitigates 1% of the design storm rainfall volume. It seems like this should read 100%. Is this a bug or am I totally missing something? Thanks!