I can't explain what's going on. There's no reason that /about/php should load about.php, in fact, it should not, under any normal circumstances.
Please check the entire configuration of your (I assume Apache) server, and paste the relevant parts, especially if it uses mod_rewrite.
Normally if .htaccess is enabled in a given directory it will automatically apply to all subdirectories. The only case where this will not hold will be if either it's been overridden by some other config, or if content is not in fact being served out of that directory at all (or for instance, by a different handler etc).
Mark