How the effect is made
The gel is painted on, then a magnet is held close to the wet surface for a few seconds. The particles line up along the magnetic field, leaving a bright band and darker surround, and the lamp locks that pattern in permanently. Every nail is magnetized individually, which is where the extra ten to fifteen minutes goes.
Because the pattern is set by hand, no two nails are ever identical. That is the nature of the service rather than a flaw, and matching ten nails closely is the skill you are paying for.