Family Tree Problem

1Great Grandma Eva 1Great Grandpa Earl 1Great Grandma Silvia 1Great Grandpa Erik
+ + + +
1Great Grandpa Steve 1Great Grandma Dora 1Great Grandpa Tom 1Great Grandma Doris
= = = =
2Grandma Cindy 2Grandpa Darrel 2Grandma Anne 2Grandpa Edward


3Grandma Cindy 3Grandma Anne
+ +
3Grandpa Darrel 3Grandpa Edward
= =
4Mom 4Dad
4Mom
+
4Dad
=
5You

So the question is, how can one be his Grandpa while being your son?

Legend
1-5:Generations
Bold: Your Fathers side
Not Bold: Your Mothers side