This phenomenon you mentioned is correct! Anyone who practices calligraphy has this experience.
When drawing vertical lines, especially when using a pen, the wrist generally does not move and is only controlled by the fingers; when drawing vertical lines, in addition to finger control, the wrist also needs to move. In this way, there is another control object when drawing vertical lines - the wrist. In probability, the more links in a series of events, the greater the probability of failure. Writing is consistent with this axiom.
So people are more likely to have problems when drawing vertical lines, so "people draw vertical lines straighter than horizontal lines."