You are looking at the intervals between the rivets and not the rivets themselves. There will always be one less interval (space between rivets) than the number of rivets. And it is the space that is 35.2 mm in length. And you have 27 of them.
Example:
X = rivet
You have 5 rivets and 4 intervals (spaces between the Xs).
Rite masters is rite, consider it like this, you have 28 rivets, and each one should have 35.2 millimeters in front of it before the next rivet, so logically, you would think 28*35.2, HOWEVER, you don't include the distance after the last one because there isnt another rivit after it.
So you have 27 instead, then u have 27*35.2= 950.4 , this is the right answer.
Hope this helped.