Put two horizontal line segments end to end with a small vertical offset between them, and a person will see the offset at a scale far below what they can resolve as two separate dots. Under laboratory conditions the threshold for spotting that offset is roughly five to ten arcseconds, against about sixty arcseconds for ordinary letter acuity. The effect has a name, vernier acuity. It's sometimes called hyperacuity, because it beats the spacing of the retina's own receptors. These are standard vision-science figures. If you plan to quote them, check a current textbook first, since the exact laboratory numbers vary with the stimulus.
That sensitivity drives the first impression a slide makes. Before a reader has parsed the title, the visual system has already compared edges across the whole frame and returned a verdict on whether they agree. The verdict arrives faster than reading, and you can't inspect it. That's how the partner in the opening could see the problem and still not name it.
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How do you measure alignment on a slide?
So alignment is measurable, and measuring it beats arguing about it. Take every element on a slide, take its four edges, and ask of each edge whether it's within a tolerance of a guide. The Slide Linter's alignment rule uses a tolerance of 0.02 inches, which is 1.44 points. It's tight enough to catch a hand-dragged box and loose enough to forgive a rounding difference in an exported file. The comparison is strict at the boundary. An edge counts as landing only if it's inside the tolerance, so an edge that's 0.02 inches out counts as a break.
The score is then one minus the share of edges that miss:
alignment score = 1 − (edges off guide) / (4 × elements)
Run it on the twelve-element slide from the opening. Forty-eight edges, 22 horizontal misses and 23 vertical misses, so 45 breaks and a score of 1 − 45/48 = 0.0625. Run it on the rebuilt version and every edge lands, giving 1.0. The two slides carry identical content. The whole gap between 0.0625 and 1.0 comes down to whether somebody dragged or typed.
What does a low alignment score mean?
One consequence is worth stating before the mechanics start. A score of 0.0625 doesn't mean the slide is 6% good. It means one edge in sixteen happened to hit a guide by accident, which is roughly what chance would produce. A hand-placed slide scores near zero on this rule for the same reason a hand-drawn circle is never round. The process has no way of landing on a value. Keep that in mind the next time a slide looks almost right.