The Problem
This is a known issue with Joomla. When you install any Joomla components thru Joomla's Admin section, all files are automatically uploaded & 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 a system username which belongs to the Apache Webserver (that's the component which is used during the uploads). This issue is already documented on Joomla's site here.
The Solution
If you are hosted on a shared server, you cannot change ownership through your FTP client or CPanel's File Manager. In that case, open a support ticket & let us know which files/folders need ownership changed. Our support team will fix this for you quickly.