More on Castle Cary charges

According to "A List of Post-Towns and Principal Places with the Postage of a Single Letter to or from London according to the Actual Routes of the Post" published for His Majesty's Stationery Office in 1830, the postage in the General Post from London to Castle Cary should be as follows (see scans below):

• To Castle Carey          9d  (in italics which I take to mean that Castle Carey was not a Post Town)
• To Shaftesbury            9d
• To Shepton Mallet     10d
• To Wincanton              9d  (The post road moved so that Wincanton became a Post Town in 1830 and took over the 5th Clause Post instead of from Shaftesbury)

This indicates that at least by 1830 the correct postal rate in my previous post should have been 9d - so why were some letters charged 10d between the middle of 1825 and 1827 ?  If you have any examples of letters from London to Castle Cary in this period (1824 to 1830) please let me know what they were charged.