Recently I took a closer look at 最後 and 最後尾, as I tend to get those wrong. I realised that the reason I kept getting those wrong was the fact the the former is read as さいご, while the latter is さいこうび. The mnemonic says to watch out as the reading for “end of a line” is different from what you’d expect, but does not really say if there is some reason for this difference. Is there a grammatical explanation? Or is it just some random accident?