The dogs learned to read fairly well, but were not interested in reading anything except the Seven Commandments. Muriel, the goat, could read somewhat better than the dogs, and sometimes used to read to the others in the evenings from scraps of newspaper which she found on the rubbish heap. Benjamin could read as well as any pig, but never exercised his faculty. So far as he knew, he said, there was nothing worth reading. Clover learnt the whole alphabet, but could not put words together.
This is a quote from Animal Farm.
I know that learnt is the BrE version of learned. There're native speakers say that these two are interchangeable, even using both of them in the same dialogue/writting is acceptable in an informal situation. As I know, however. Orwell is a careful writer, hence the inconsistency in his book, and what's more, in the same paragraph should be deliberate. So I want to know the reason he did it, do these two words have different meanings or any difference in nuance?