Imagine you are the clerk recording in long hand all the details for the passenger lists, and you know that the vast majority of the men list their occupation as Miner, as that is what is needed in the colony. What's the easier option, without first stopping and wandering off to find the passenger to check first hand if he is a Miner or a Shepherd .....
Imagine you are Geo NIXON standing at a counter seeking a ticket to Australia to 'make your fortune' finding Gold, even though your upbringing and training has been as a Shepherd. The ticket clerk says "occupation" .... do you say I am a shepherd, or do you say "same as the others, I dig gold".
He lands in Victoria, gets to the goldfields, tries his hand, doesn't get much return for his efforts, but sees there's many groups of people who want food on their table. He finds an employment bureau and discovers that farmers are paying regular wages for shepherds, and of course a shepherd has butchering skills, so he can take up his former occupation here in this new land.
Speculation of course, but likely not very far off the mark.
Remember that the passenger lists are not in the handwriting of the passengers, and if the info is recorded in the presence of the passengers, for it to be validated by the passenger, the passenger needed to be able to read, and to read well .... in fact to read long hand writing 'upside down' for the document would be facing the scribe, and not the passenger.
JM