Dunster Fifth Clause & Penny Post - postal charge problems !

In 1826 Dunster was still a Fifth Clause Post of Taunton.  The entire below has the boxed "No.4" handstamp (only known used in 1826) along with a Dunster mileage mark.  I originally thought that it should have been charged 1d for the 5th Clause Post to Taunton plus 4d for the up to 15 miles from Taunton to Wiveliscombe (ten or thirteen miles depending on whether it went via Wellington) making 5d - not the "7" which is clear on the front.

However, on examination of other Taunton Fifth Clause Post letters, they all seem to either omit the 5th Clause Post charge (correctly, because the sender is supposed to pay the 5th Clause Post charge) or they charge as if the whole distance had been in the General Post - which is what has happened in this case.

In February 1829 Dunster was made a Sub-Office to Taunton so postal charges to/from Dunster were based on the General Post rate rather than being in the Taunton Fifth Clause Post.

The entire below has a "Dunster / Penny Post" handstamp and is charged 6d to go to Wiveliscombe. If the entire went via the route I would expect, that is from Dunster to Taunton and then back out to Wiveliscombe, the distance is around 30 miles (actually a little over) which would translate to at most 6d in the General Post rate (probably 7d as in the entire above) - this does not leave anything for the Dunster Penny Post.

Without the Dunster handstamp one would imagine that it would have gone from the Washford Receiving House to Taunton for 1d in the Taunton Penny Post, and then been charged 4d in the General Post for the less-than 15 mile from Taunton to Wiveliscombe, making 5d in total.

Can anyone give me a clue why the first entire is charged based on the General Post all the way and why the second entire has the Dunster Penny Post handstamp and is charged so little ?