'Easy to Read Fonts'?
Arial isn't easier to read than Bodoni. In fact, it's the other way around for any longer body of text. Sans-serif fonts such as arial are often only easier to read when used at display sizes, in headings for example, or given enough leading to allow plenty of whitespace between lines. This is because the tops of the lowercase character shapes (where your are tends to skim when reading) and width of strokes are very similar in a font such as Arial, but offer more variety with a serif font, particularly one with such pronounced thicks and thins as Bodoni.
And that is why most books are set in serif fonts, because it's much easier for the eye to recognise the word shapes.
The actual difference here isn't the font per se, but more likely the difference in contrast. If you give someone two pieces of type to read using identical font, point size, leading, kerning, tracking etc they'll be able to read the piece that's 100% black on a white background much more easily than the piece that's 60% black on a white background, because of the greater contrast.
The controls for this test should have been a group using the 60% black on white set in Arial and a group using 100% black on white set in the other fonts (Bodoni and Comic 'crimes-against-typography' Sans) to check whether the results were truly reflective of the chosen fonts or simply the visible contrast in the supplied material.