Alignment of cols

If I look at the page http://firezenk.github.io/zimit/design.html, it looks as if the 4 cols actually span 5 cols including the margin of the fitth column (if you compare them to the 12 columns above). And both 4 cols spans end somewhere around the middle of the 11th col. Now, I find that sort of math difficult. I would expect 4c to span 4 cols, and then one margin width, and then another 4c to span 4 cols ending up covering a total span of 8 (eight), as in 4 + 4 = 8. I don't get how 4 + 4 = 10,5?