Due to gravity and the physics of extrusion, larger pipes tend to be slightly thicker on the bottom than they are on the top. However, the top will always be at least as thick as the production SDR, so there is no concern over pressure and temperature ratings.
This difference is not enough to cause problems with the flow calculations or to require a change in fusion pressures. The only concern is simply an aesthetic one, where the internal bead will be misshapen if a thinner top is fused to a thicker bottom. To avoid this, line up the tops and bottoms of the pipe before fusing them. The easiest way to align the pipes is using the printed label on the side, as the label is always in the same relative position to the top of the pipe. Aligning the labels will help eliminate internal misalignment. If aligning the labels does not fix this issue, use your best judgment when aligning the pipe. The issue does not affect the outer wall of the pipe, which will always be consistent.
View PDF: 201211A-AQTTB-Internal-Alignment.pdf