70 is obviously exactly half of 140. It's easy to argue that a track in the tempo of 140 could be seen as a track in the tempo of 70, just with notes that are half the length (i.e., a bassdrum kicking in eights instead of quarters).
For this reason, many BPM detection algorithms need to be given a range, such as "90 - 179", which does not contain any pair of numbers with one being a multiple of the other.
