The distances between the towers measure within 1mm from each other, the heights are as well, the rods are identical by way of assembly.Quote
A2
Deltas require symmetry, parallelism, and squareness.
Error of movement comes from these deviating from being perfect, (assuming your software variables are not mucked up).
Other reason i don't think it's a problem of geometry or assembly - the problem is directional, not an absolute deviation or something you would expect from a bodged geometry.
If it goes one way, it curves up, the other - goes flat.
I'm using Marlin firmware from GIT of a week ago.