According to Kawafumi on Discord: "it's about the 'reverse' mod not being accurate, but it's worth mentioning again that k-pump has two mods that flip the gameplay area and neither is equivalent to 'reverse' anyway:
'Under Attack' flips everything 180 degrees
'Drop' flips the gameplay area vertically, including the graphics"
stepmania/stepmania#103
According to Kawafumi on Discord: "it's about the 'reverse' mod not being accurate, but it's worth mentioning again that k-pump has two mods that flip the gameplay area and neither is equivalent to 'reverse' anyway: