Hi,
Looking at your case otherwise, assume that the thickness be uniform around the cross-section of the bend.
In this case, for a same (assume small) included angle,
1) inner curvature called CROTCH or INTRADOS, has less area at disposal (or exposed) to resist the pressure force.
2) Outer curvature called CROWN or EXTRADOS, has comparitively more exposed area at disposal to contain the pressure force.
As a result,
due to differences in area, more Hoop stress is developed at intrados than that at extrados.
Meaning to say, for the SAME ALLOWABLE STRESS, a thicker wall will be required at the intrados and a thinner wall can be used at the extrados.
Now, this thinner wall thickness mentioned above is what seems worrying you and i hope now you have your answer as you can now say :
as Hoop stress in a curved segment of pipe varies around the circumference of the cross-section, one need to have different thickness.
_________________________
Dinesh K.