This is a known issue with Joomla. When you install any Joomla components thru Joomla's Admin section, all files are automatically upload & installed by Joomla. But the problem is, Joomla still keeps the file ownerships set to "nobody", instead of changing the ownership to your CPanel's username. "nobody" is actually a system-username which belomgs to the Apache Webserver (that's the component which is used during the uploads). This issue is already documented on Joomla's site here.
If you are hosted on a shared server, you will NOT be able to change the ownership yourself thru your FTP client or CPanel's File Manager. In that case, simply open a support ticket & let us know which files/folders needs to have ownership chnaged. Our support team will filx this for you easily.