The former includes "re-reads", e.g., if you've paged back to a page you previously spent time on.
The latter only accounts for the *actual* amount of pages in the books (and, for each of those page, re-reads are coalesced and capped at the per-page max).
This means that it's entirely possible for both to be equal, but "total" can *never* be lower than "reading"

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