It's not due to WebDirect but due to the fact that global field values are specific to each user when the file is hosted over the network. This is true for FileMaker Pro and WebDirect clients of your database.
See this Knowledge Base article on the subject: http://help.filemaker.com/app/answers/detail/a_id/3604/kw/global
There are ways to manage this issue that aren't terribly difficult, but you'll need to describe how you are using these global fields first.
Ok I think I have misunderstood how global fields work.
I have created some fields, which I want to have a certain value. I want this value to be the same on all records.
Is there a way of doing this?
Yes you can probably use global fields, but just control how you assign those values. If, for example, you take the file down off the server, open it in FileMaker Pro and assign values to the global fields, then return the file to the server, all users will see the values you assigned as the initial values of these global fields each time they open the file.
You can also use a "server side" script--run from a schedule or via "perform script on server" to change the values of the global fields and users will see these new values the next time they open the file.
Another option is to set up a utility table with a record (with fields that aren't global) for storing the values you want to have in the global fields. Any time that you want to change the value of a field, you edit the corresponding field in this record. A start up script performed with the OnFirstWindowOpen trigger can copy the values from this utility table into the global fields of your solution.
And you can also set up a single related record by using the Cartesian Join operator (x) instead of the default (=) operator. The data in this record will be the same for every record in tables linked to it in this fashion, but all users will able to see and edit the same value.