Confirmation that prime lenses provide sharper images than zoom lenses?

I have often read that prime lenses provide sharper images than zoom lenses. Last week, for no particular reason, I used my 90mm lens to shoot some indoor (non-macro) shots. The results were amazing. During post-processing I saw a much sharper image than usual. That led me to take some outdoor shots. The result was the same. Although I did not complete a scientific comparison of my "workhorse" 24-120mm zoom lens with the 90mm macro lens, I wonder if this isn't confirmation of a much-stated fact.