According to the company, according to legend, after the founder's death, his two sons took over the business. They wanted to rebrand the bottle to enter the bitters into competitions. One redesigned the bottle, the other redesigned the label.
It was either too late to fix the mistake or they were too stubborn. One judge advised them to keep it, since it would be eye-catching for customers. They kept the glitch.

Regardless of the original reason, it keeps drips from running down the neck all the way to the bottom, which can stain surfaces with surprising tenacity.