[nine_k]

What's mysterious here? One ordering is dictated (arrival time), another left for the consideration of the server (likely allowing for stuff like pinned messages, etc, that break the strict ordering).

If a Matrix server allows to delete messages (by the poster or by a moderator), then increasing IDs with no gaps become impossible. If the server allows editing of existing messages, then a sequence with no gaps is not sufficient to reflect all changes. Ideally a server does not do either, but uses more messages to augment existing messages, or mark some as deleted; with that, a sequence with no gaps would suffice.