רש"י maintains that קרובות are always טמא, since she can bury by herself. רחוקות require an escort. Therefore חדשות are טהור for the escort would inform us of any burial; however by ישנות any report from the escort was forgotten.
תוספות agrees with רש"י concerning קרובות. However, רחוקות ישנות are טמא because we are concerned that they were once קרובות and the woman buried by herself without an escort. רחוקות חדשות are טהור for an escorted woman buries in the cemetery.