Offset is an answer to the question "where does Nth memory location start from?". The answer is "after N-1 locations". It's the count of locations that need to be skipped by the reader, to reach the start of Nth memory location.
I don't know but I feel like you are making a point out of something arbitrary. When I listen to an audio book, everyone always says: "Chapter 1", not "the first chapter" so why is this important?
I think extreme attention to to arbitrary meaningless details is how we ended up with most rules in language that we are starting to collectively detest.
Book chapters and page numbers are not offsets.