It's a mixture of the first two options; but what mathematicians allow themselves to do in the privacy of their own specialism is up to them. The perpetrators are making a false analogy with 'onto/into/unto'. On the other hand, as fivejedjon said recently in another thread, 'today's slip is tomorrow's good grammar' (or words to that effect).
PS and 'UPTO 50%' is even worse; it almost always means 'a maximum of 50% off the price of a few things nobody wants'. ( - in case you hadn't noticed)
Interested in Language