Why are Clear Art's designs more visually accessible?

Design information:

Web Content Accessibility Guidelines (WCAG) refers to a success criterion of 1:4.3 as a minimum of contrast accessibility. All Clear Art designs are AAA WCAG 2.0 compliant in terms of contrast in relation to large print contrast recommendations (i.e.4.5:1 or more). This means that the tonal difference between the image and the background is sufficient enough for most of those with severe sight loss to see clearly. Clear Art also tries to use more simple designs to avoid the visual difficulties with visual clutter.

Please see https://webaim.org/articles/contrast/

“In WCAG 2, contrast is a measure of the difference in perceived "luminance" or brightness between two colors (the phrase "color contrast" is never used in WCAG). This brightness difference is expressed as a ratio ranging from 1:1 (e.g. white on white) to 21:1 (e.g., black on a white). To give a frame of reference, on a white background:

Pure red (#FF0000) has a ratio of 4:1.
Pure green (#00FF00) has a very low ratio of 1.4:1.
Pure blue (#000FF) has a contrast ratio of 8.6:1.

Typical Clear Art Colour contrast ratios against a white background range from 4.51 to 14.07.

Image of typical Clear Art design colours