I’m not exactly sure either but that is around the time I got started with css and I can still recall a lot of these 16px conversion ratios[1], they must have hade some significance.

Edit: now that I’m thinking about it, I (used to) use Spotlight for the conversions which was introduced in Mac OS X 10.4, released in 2005.

[1] 62.5% for 10px, 87.5% for 14px, etc.