Why has 88:31 become the standard ratio for micro bars?

A recent Hackernews article linked to http://cyber.dabamos.de/88x31/ with some animated gif banners from the 90s. Which got me to wonder: how did the ratio of 88:31 become a standard size? People typically like to round to whole numbers (for either the actual size, or at least for the ratio), or we'll use a whole fraction of typical screen sizes of the time.

Excepting 88x31 size "micro buttons" a.k.a. "micro bars", just about all "standard" sizes for web banners listed by wikipedia have some kind of "round" number for at least one of the dimensions (often both actual size and ratio).

What is the origin or rationale behind 88:31 for microbutton web banners?