Interesting…both are based on the reference and could be correct. Which part are you confused about? Either way the strength of this illusion is inconsistent at best.
However, differences in the strength of the vertical-horizontal illusion or the related Müller-Lyer illusion for these groups are inconsistent at best. — En.wikipedia.org. (2017). Vertical–horizontal illusion. [online] Available at: https://en.wikipedia.org/wiki/Vertical%E2%80%93horizontal_illusion[Accessed 24 Nov. 2017].