Naming a parent directory as a number

Hi Guys, can’t find an answer anywhere, but may be a tip for other people… i have created a new wordpress site, but when setting the post by name, it could not write the .htaccess file, also i kept having to reset my password everytime i logged out, tried installing with different databases etc and happened everytime, but then i changed my parent directory from a number (121) to text (onetoone) and everything is perfect.

My question being, it is obvious you cannot use a number (or 121) as a name for the parent directory, can anyone explain why?

Thanks in advance… ??