Mine has a light gap between plastic back and metal frame(about 1/10mm or less, noticeable only after a very maniacal and close inspection)from micro sd slot to micro sim slot.
The back is not perfectly flat, but isn't a metal frame bending: if placed face down on a table the entire screen touch the surface, leaving face up the corner next micro sim does not touch the surface (barely noticeable: 1/10mm or less). The phone remains waterproof (at least totally submerged on 10cm of water).
Maybe there are few minimal tolerance using flexible material as polycarbonate back, when placed on direct contact of a non-so-flat thing as a battery.
Li-po and li-ion battery doesn't have a surface totally flat, specially on the borders and edges: there are frequently minimal differences and all phones with removable battery have a small gap between battery door and battery filled with some lining to compensate. On Z1 Compact teardown we can se the battery on direct contact of policarbonate rear.
Attenction: this i not a defective battery: a damaged lipo has central rise of several mm not a lateral of 1/10 - 1/20 mm; on side there are simply the "bend" of aluminium sheet and the adhesive label encasing the li-po cell.
The phone works well, looks great and retain the water proof capability.
We are all to apprensive with our new toy