Still not explaining "why", but Wikipedia suggests that it was part of a list of standardized sizes, this one being a "micro bar": https://en.wikipedia.org/wiki

Still not explaining "why", but Wikipedia suggests that it was part of a list of standardized sizes, this one being a "micro bar": https://en.wikipedia.org/wiki/Web_banner#Standard_sizes https://en.wikipedia.org/wiki/Web_banner#Standard_sizes