I know that someone linked me to that head banging against a wall gif once, but I can't find it now, when I really need it.
1) That chart above leaves a lot to be desired... probably produced by someone to "get a chart online" in a hurry. Here's a better chart, taken directly from their dealer catalog... it should help explain things better...
Note that Reach plus Scs does NOT equal O. That is because reach is measured at the center point of the very top of the head tube. Whereas O is measured from the center point of where the top tube and the headtube intersect. Because the headtube is angled those two places will not be the same. Reach can be a deceiving number sometimes for some people and when comparing between bikes it is only a comparable number if it is measured at exactly the same stack height. As it is, Reach will always be a function of headtube length. People seem to forget that, or never think about it a lot of the time. A good "reach" number to be measuring is the reach at the stack height that you ultimately want your handlebars to end up at, which would take headtube length out of the equation. You certainly want to know headtube length when it comes to determining how many spacers you will need, however.
But the above only explains part of the difference that @cadence90 is wondering about. The other part is that Scs is measured at the actual seatcluster center, NOT the effective seat cluster center which would be higher and further back. So, they show the effective top tube length (O), for a sloping geometry, but they only show the actual setback of where the actual seat cluster sits in space, and for a sloping frame that spot is a fair bit lower and further forward than a traditional frame of equivalent size. Hope that clears things up a bit, or maybe it just confuses things more.
So, having said that... here's a more accurately drawn chart...
And here's the entire geometry chart, although it doesn't have headtube angles which are currently shown on the charts on Colnago's website...
Have fun.... and Happy New Year!