There are lots and lots of reasons. Sometimes authors or publishers fix errors between editions. That could be because an inconsistency within the book, or because they found something out later. The First/Business class change could be either. Maybe because of something else in the book it is inconsistent. Or maybe the author found out that that particular plane/flight/airline didn't have that class and decided to fix it.
Another reason is the nationality of the publisher. "Harry Potter and the Philosopher's Stone" was published as "Harry Potter and the Sorcerer's Stone" in the US. And there were other changes to the language used (see
https://en.wikipedia.org/wiki/Harry_..._and_reception for some examples). That is not an uncommon thing. I have seen at least one book where they "corrected" spelling (colour vs color). And, as you are in Australia, there is a good chance your paperbacks were source from UK based publishers and your ebooks are being sourced from US based publishers.
Another reason, is that the publishers edited the book, or forced the author to, and later the author managed to get the original version published (Robert A Heinlein's "Stranger in a Strange Land" is an example, though the original was published posthumously). Or different publishers did different things. That seems to be the case with "Imperial Earth". According to
https://en.wikipedia.org/wiki/Imperial_Earth#Editions, the US and UK editions are different. There is no explanation of why they are different, just that they have different chapter counts.
It could be the publishing format and the author adding things when that changes. An example, look at,
https://en.wikipedia.org/wiki/Rochew...cation_history. Originally serialised and the author extended it when published it as a book (but, maybe the magazine edited their version). The author extended it twice after that for new versions with a title change at the end. I know I borrowed one of the versions title "Flight of the Dragonfly" from a library and read it. Then stumbled on the final version later. [For the record, a good book but not great characterisation. But, the science is very good. Robert L Forward has some of the most interesting, and possibly buildable, propulsion systems for space craft ever. And his aliens tend to be unique.]