How does the Craft Grid System work?
This is somewhat of a guess, but is based on what I could find. It might not answer everything but it might be a start :)

From what I can work out the grid work is worked out around the values you provide yourself, if you look at app/resources/js/craft.js on line 10244 this seems to be where the grid starts.

On line 10339 is the refreshCols function which seems to be responsible of figuring out how wide items are are where they should be placed.

It looks like Craft will take the values you supply data-max-cols="3" data-min-colspan="2" etc and try to work out the best possible layout (This is where data-position gets dealt with aswell) ...

line 10466
// Figure out all of the possible colspans for each item,
// as well as all the possible positions for each item at each of its colspans
...

line 10532
// Now find the layout that looks the best.
// First find the layouts with the highest number of used columns
....


etc etc, so it looks like there aren't really any set defaults, you just gotta play around with the numbers and see what works best for the layout you are trying to achieve.

Lastly, on line 10775 is:

defaults: {
    itemSelector: '.item',
    cols: null,
    maxCols: null,
    minColWidth: 320,
    mode: 'pct',
    fillMode: 'top',
    colClass: 'col',
    snapToGrid: null,
    onRefreshCols: $.noop
}


I realise I could be looking in the wrong direction and apologies if this doesn't completely answer your question, but hopefully it will provide an insight.