There are many varieties of English pronunciation. The existence of different varieties has implications for English spelling. A paper by the retired phonetician John Wells discusses some of those implications. https://www.phon.ucl.ac.uk/home/wells/accents_spellingreform.htm
One of those implications arises from the distinction between rhotic varieties and non-rhotic varieties. In the following positions, rhotic varieties pronounce the sound /r/ but non-rhotic varieties do not pronounce that sound:
- before consonants (eg in farm)
- at the end of word (eg far)
The paper by Wells notes that speakers of non-rhotic varieties need to learn which words are spelled with the letter <r> in those positions. This is because their own native pronunciation does not give them that information.
In a pun that may well be unconscious, the paper says that non-rhotic speakers must learn this distinction by rote.