Its because you can define several active designs for a siteaccess, witch lets you have multiple fallbacks.
Normally you'd have something like
1.'your_design' (override for your custom site)
2. flow (if you use ezflow)
3. webin 3. standard
And for admin you'd have:
1. admin 2. standard
This is configurable using the setting mentioned above by Stéphane, as well as the AdditionalDesign setting and StandardDesign setting in site.ini.
So in the example you mention you have 'placement overrid' in effect, and there is no need to use any overide.ini condition to make it work. override.ini overrides are for conditional overrides(like for a specific node or class), placement overrides are for global 'override'.
Thank you very much fo your answers, in my configuration I have
[DesignSettings]
SiteDesign=ezwebin
so I think that ez normaly must go search in www\my-site\design to find a folder named "ezwebin" but in this path (www\my-site\design) I don't have this folder, so he look for the folder in www\my-site\extension\ezwebin\design\ezwebin.
1/ he look in www\my-site\design and when ez don't find "ezwebin". 2/ he look for ezwebin in www\my-site\extension
1 design (like 'standard' or 'ezwebin') can have several placements, the order of this kind of fallback is:
1. design in extensions in the order the extensions are loaded 2 design/ folder