The classification for my bachelor's degree was only based on the final two years. However, the WES report reflects a different method for calculating the GPA. Why is that?

We are aware that final classifications for British undergraduate degrees may be based on only part of a degree program. However, WES calculates a cumulative grade point average (GPA) based on all subjects, all grades, and all years studied as this is the method used by institutions in the United States and Canada.