Result of complementation analysis:

Question: How many complementation groups does the analysis
reveal? Divide the mutations in complementation groups.
My answer would be 4 complementation groups:
Group 1: mut1
Group 2: mut2, mut4, mut6
Group 3: mut3
Group 4: mut 5
However, the textbook answer is 3 complementation groups:
Group 1: mut1, mut5
Group 2: mut2, mut4, mut6
Group 3: mut3, mut5

How can mut5 belong to 2 different complementation groups?