6/12 is correct because the scoring is done vertically and horizontally, not diagonally. So you can score a maximum of 4 vertically and 2 horizontally, or 8 and 4 (counting the Pavements building in the vertical line). Or if you choose to count the Pavements building in the horizontal line, it would be 3 + 3 / 6 + 6.
Pavements (Counting buildings, not showing points here)
Drainage (Counting buildings, not showing points here)
The example on page 15 of the rulebook shows how to score the buildings. The one showing the Pavements scoring example is illustrating that gaps are allowed, not that 5/10 is the maximum.