Still there after some were taken. What is left is measured against what was.
Built on remain, from Old French remaindre, from Latin remanere (to stay behind): re- (back) + manere (to stay, to remain); what is remaining has simply stayed behind while the rest moved on or was taken.
Still there after some were taken. What remains is measured only against what there used to be, exactly as the root's staying-behind implies.