The first backtracking is possibly to look again at the theory that is being pointed at. The second backtracking is due to a less frequently used word (constancy).
b. Fixations are longer at words : supported, constancy and Lamarck
c. The general reason for longer fixation could be the length, frequency of use and familiarity with the word. I have not fixated at the numbers at all which indicates that figures are easily comprehended than words.