Measuring centre to centre can confuse you.

The measurement that matters is the distance from the face closest to the end of kayak of one form to the face closest to the end of the kayak on the next form

In other words you are measuring from one point where the strip touches one form to the point where it touches the next. That is the outside edge. The strip doesn't touch the centre of the strip (except right in the middle of the kayak), so don't measure there.

If form 3 fits against the end of the form then it should be 12" from outside edge to outside edge.

My impression of your photos is that if you put form 2 between the lines, then that measurement will be correct. Unless the perspective of the photo is didtorting the view.