The reason this happens is on a Unix based box many setups of apache runs under a different account then your FTP access and that process user OWNS the files it writes to disk. Your hosting provider should be able to fix this but on some of these shared hosts that might not be the case.
JoomlaExplorer is the best "fix" for this if your hosting provider can't help you.
Magento - phpBB3 - Kunena - RokBridge Specialist
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