A steps per distance error would not be cumulative like that.
If the machine were moving say 10% further than actually commanded then a 1mm move command would result in 1.1mm move. This would mean 1.1mm between each hole center to the next, though overall after 5 holes there would be .5 error (moved 5.5mm).
The opening post describes incremental error between each hole. (1mm spacing at start becomes 1.8mm by end)
Check all mechanical connections. Tighten screws. Put a line mark (sharpie/texta) on the ball screw and couplers to see if they are slipping
7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)