Arve Bersvendsen has written an interesting post in his blog about misleading browser stats. As a quick summary, Opera's aggressive caching could lead to fewer hits compared to other browsers with less aggressive caching (IE and Firefox). This is even worse for text-only browsers like Lynx, as they typically don't request external style sheets and images at all.
Read more in Arve's post about misleading browser statistics.
In addition, Opera identifies as MSIE by default, and even though "Opera" is part of the useragent string, many statistics sites fail to detect Opera. In Opera 8, it is even possible to completely cloak Opera, and that means that statistics for those sites show no Opera users at all.