Usually, the ISBN number is supposed to change when the edition is modified.
The modifications may be minor and not matter much to readers.
The publisher is not supposed to change the text, except for typos and such,
though that could depend on contractual agreement in some countries. It may
also happen that some parts of the text have different versions, all produced
by the author. These issues become very complex, but matter only to scholars.
Are you missing something obvious? Well ...
You seem to be looking in various places for your ebook. Possibly you are not
aware that these places are not equivalent. Ebooks come in a variety of
formats, mostly epub, pdf and mobi. These have variants, but that is less
important. Actually there are lots of formats, but you are very unlikely to encounter most of them.
The mobi format is used only by Amazon for the kindle, while nearly everyone
else uses mostly epub (and sometimes PDF, though PDF is not too convenient for
that purpose, because less adaptable).
Thus, when looking for ebooks, you have to know on what device you intend to
use it. If you use a computer, you will find applications for reading any of
the three formats. But if you intend to use an ebook reader, then you have to
buy ebooks in a format accepted by your e-reader.
I personally prefer epub which is an open standard. In all things technical, I
hate to be tied to a single provider, no matter how large and powerful.
Another issue you should investigate is DRM. DRM are software devices that
control your use of books, whether you can have them on different devices,
whether you can lend them or make copies. It has occasionally been used to
erase books in the home of the owner (just to give you an idea of the
potential of DRM). Actually, the book that was thus erased from its owners was
precisely a specific edition of 1984.
I tend to avoid books that carry DRM, but it is too often the case that a
given title is available as ebook only with DRM. However, there are cases
where a book is available with DRM from one retailer, while another retailer
will sell it DRM-free. Sometimes it is a matter of publisher, and the same
retailer is selling two versions of the book, with DRM from one publisher, and
without from another publisher.
Some form of DRM are somewhat innocuous. For example watermarks, are quasi
invisible markers that identify you as the owner so as to be able to trace
illicit copies. But it will not interfere otherwise with your use of the book.
Last point. If you want better precision about the book size, use word count
rather than page count. You often get both.