I'm stumped, I can't figure out what's going on here. I've got a lattice and a plane with identical x & z dimensions. When I use a Simple Deform Bend of 360 degrees on the lattice, the edges meet seamlessly to create a cylinder - but the edge of the plane don't. It's as if there's a gap being created between the edges of the plane and the edge of the lattice.
All transforms are applied apart from the y scaling on the lattice. Changing that back to 1 doesn't change anything, and neither does moving the empty I'm using as the origin for the deform. The solidify is just there to show what's going on from the top view - removing it makes no difference.