Hi Michel,
The web.base.url works by design like this. Everytime a user logs into your Odoo instance Odoo will check if the web.base.url that is already saved is identical to the URL that the user logged in from. If this is the case it is kept, if this is not the case it will be updated. This might get confusing when you have your Odoo instance available on two URL's, for example url1.com and url2.com. If somebody logs on from url2.com the web.base.url will be saved as "url2.com". The same will also happen when you access the Odoo by IP. In this case the web.base.url will be saved with the value from your IP.
There are two possible solutions. EIther you make sure the Odoo is only accessible through that one URL you want (so yourwebsite.com) or you make sure Odoo does not change the web.base.url. You can make sure that this does not happen by adding a new parameter "web.base.url.freeze" with the value "True". An example:
The web.base.url.freeze will say to Odoo that the value cannot be changed/update when somebody logs on from another location. It will keep the value that you've set in web.base.url.
Regards,
Yenthe