Back in the early days of the internet, a lot of websites were hosted on Geocities, which was popular as it offered free hosting on one condition - you had to embed a small banner into your webpage advertising them. The banner image they provided was 88x31 pixels, and so many Geocities sites would include other external links as 88x31 images so that they matched the dimensions of the mandatory Geocities one.
At the bottom of the stack, the answer is probably either "it fit perfectly in an 88x31 space between some other things on the first page it was used on" or "I drew a rectangle that looked right and didn't bother rounding the numbers".
Back in the early days of the internet, a lot of websites were hosted on Geocities, which was popular as it offered free hosting on one condition - you had to embed a small banner into your webpage advertising them. The banner image they provided was 88x31 pixels, and so many Geocities sites would include other external links as 88x31 images so that they matched the dimensions of the mandatory Geocities one.