I'm a little confused
a phenotypic ratio for a dihybrid cross is predicted to be 9: 3: 3: 1. the experimental values are purple smooth = 75, white smooth = 28, purple wrinkled = 24, white wrinkled = 8. account for the difference between the expected and experimental values.

The differences observed between the expected and experimental  phenotype is purely due to chance.

Explanation:

First, there is need to check if the expected and observed outcome are significantly different using chi square.

                               Observed phenotypes                  expected phenotype

purple smooth                  75                                         9/16 x 135 = 75.94

white smooth                    28                                          3/16 x 135 = 25.31

purple wrinkled                 24                                          3/16 x 135 = 25.31

white wrinkled                   8                                             1/16 x 135 = 8.44

Chi square =

for purple smooth = = 0.012

white smooth = = 0.29

purple wrinkled = = 0.052

white wrinkled = = 0.023

Total chi square = 0.377

Degree of freedom = 4 - 1 = 3

Tabulated chi square at 95%, 3 df = 3.81

Hence, there is no significant difference between the expected and the observed outcome. The differences observed in the phenotype is purely due to chance.