Some manufacturers measured frame size from the centre of the bottom bracket to the top of the seat tube (c-t) and others measured from the centre of the bottom bracket to the centre of where the top tube (tt) meets the seat tube (c-c). If I remember correctly, Marin did the latter, so a 17" Marin could be about 19" if measured to the top of the seat tube. The top tube on a 17" (c-c) 1995 Pine Mountain is 57 or 58 cm (I can't remember which and I no longer have the bike).
The horizontal measurement is the top tube measurement from the centre of the head tube to the centre of the seat tube. (Now that most top tubes are sloping, the measurement for a 'virtual' horizontal top tube is more often quoted.) Entry level bikes tended to have slightly shorter top tubes, giving a more relaxed riding position; higher end, racier bikes tended to be a little longer, giving a longer and lower position.