I'd expect that to be the case. Because fares in London are zonally based you pay for 3 zones from Mill Hill but only 2 zones from the boundary.Pretty sure when I purchased a BZ4 to Elstree & Borehamwood (the next stop up the line) it was cheaper than buying a Mill Hill to Elstree & Borehamwood but I'll have to go back and check my old tickets.