Error_reporting is also set from Joomla through error_reporting (value_from_configfile).
If your config overrides this, you might want to change that behaviour.
You can change your php.ini accordingly to avoid this. This change is made in php.ini something like this: $error_reporting in confid.php file:
Default values look something like this:
error_reporting = E_ALL & ~E_NOTICE ; Show all errors except for notices
display_errors = Off ; Print out errors (as a part of the output)
You may also need to run Debug to see if something more specific is causing this.
error_reporting = E_ALL | E_STRICT
display_errors = On
I hope this helps you to get started.
Joomla.org will have more info on this if Google is no friend to you.