Hi there
In your first example, if you access "/matches/" the second rule is redirecting you to your about page. This is because ^([a-zA-Z0-9]+)/$ is trying to find any combination of letters and numbers and a slash, which includes the string "/matches/". If you want something else to happen when you access "/matches/", create rules like this:
RewriteRule ^matches/?$ ?k=matches
RewriteRule ^about/?$ ?k=about
The question mark is useful before the slash as it means with or without the preceding character. Hope this helps.
Jamie