On Saturday I went Portlethen-Aberdeen-Stirling. Portlethen is about 8 miles south of Aberdeen so I did double back on myself. On getting on the 1111 Portlethen to Aberdeen train I bought a Portlethen to Stirling return for £41.70 from the conductor with no issues and I then got the 1142 to Stirling and all was fine. On returning I got the 1907 from Stirling and was looking to get into Aberdeen for 2122 and have a BoJ there before completing my journey on the 2227 to Portlethen. When I tried to exit the barriers in Aberdeen my ticket wouldn't work and when I spoke to the CSA they told me I had short fared and I had to buy a Portlethen to Aberdeen single at £3.60. On looking at the Scotrail site it says that if I had got the 2007 from Stirling going to Aberdeen and then doubling back to Portlethen is valid so why isn't it valid to do it on the 1907? Ultimately I'd have been getting on the same train to complete my journey to Portlethen.