I think the price was doubled to tuppence and the extra used to fight Napoleon, but I don't suppose that explains why long or short letters were wrong.
I don't know about during the days of the Penny Post but prior to the Penny Post when letters were sent the price of postage was calculated on the distance a letter had to travel.Postage was paid for on receipt of the letter .They used what were called Bishops Marks .If you were poor and received a letter and couldn't afford to pay you didn't get your letter .Or people could just refuse to accept the letter .
This was one of the reasons Rowland Hill campaigned for reform of the postal system .
Look him up ..it may be something to do with weight and distance .
Actually Rowland Hill was not the inventor of the penny post but William Dockwra had already established the penny post 240 years earlier.
It handled packets of upto one pound in weight and delivered within ten miles of London for an extra charge of one penny he was forced to give up his business in 1683 when the government started the General Post Office. In 1764 Parliment authorised the creation of the Penny Post in any town or city in the UK by the 19th century there were several in existance.
in 1840 Rowland Hill established the Uniform Penny Post soon after it could be prepaid with the introduction of the Penny Black. In 1898 the Impererial Penny Black extended the rate throughout the British Empire. Queen Victoria popularised the post by starting to send greeting cards and like thae fact that her portrait appeared on them.
Britain being the inventors of the stamp is the only country not to have its country of origin printed on the stamps.
Post actually goes back to 2400 BC asc far as we know.