What is in-plane fluctuation?
In this real world, nothing goes ideally. Your printer doesn’t work ideally, too. In most cases, your eyes cannot find the divergences. It’s not a coincidence. It’s an art. The art of printer manufacturers.
Let’s see one of such divergences.
On Photoshop, make a patch of 6 mm (1/4″) square of solid black, copy and paste it a lot horizontally (along to printer head moving direction), and print the image by an inkjet printer. Your eyes cannot find any fluctuation among those patches. But, scan the paper by a photo scanner, and inspect it by Photoshop. It fluctuates. In many cases, the amplitude of the fluctuation is 1-2 by ΔE00. The behavior comes from the thixotropy of ink. Ink is viscous after tranquillity, and it is runny after disturbance. Ink ejection is disturbance for ink.
Some expensive photo printers perfectly prevent this effect. But yours? If an ICC profile builder doesn’t account this effect, its result becomes miserable, especially in terms of tone reproduction. Many existing ICC profile builders fell into this pithole. Moreover, the fancy precision of colormeter doesn’t make much sense for such common printers.
Of course, Zygomatic Color handles this problem well.