Due to the offset between the cartridge's axis (which is approximately tangential to the disc) and the tonearm's pivot, the force applied (through friction) by the rotating disc to the cartridge tends to draw the tonearm toward the center of the record and distort the balance of the sound and of the wear suffered by the stylus.
You are correct that skating force is maximum at the outer groove. It then decreases gradually to the lowest point on the blue curve. After than it increases to the innermost groove, but it never quite returns to the maximum level of the outer groove. Skating force is proportional to the blue line with approximately 11% reduction at the bottom of the blue line.