BuiltinAdminsAreServerAdmins is a server property in Analysis Services which is set to true by default as shown in the figure below.
What this means is, all the local administrators on your windows machine are going to be server administrators for analysis services. In other words, windows local administrators will be members of analysis services built-in Administrator role. But the point to note here is that, when you look at the security page in the server properties, you will only see users who have been explicitly added to the Administrator role. For example, on my server, only the windows Administrator is explicitly added to the analysis services built-in Administrator role as shown in the figure below.
However, I have a couple of users who are members of the windows local administrators group as shown below. So in my case, users zack and GMiller will have implicit but absolute control over analysis services.
This is a bit risky, especially on a production server. I recommend turning this property to false to prevent unwanted users from administering analysis services. Hang on, before you go and change this property to false, please note that when you change the BuiltinAdminsAreServerAdmins to false, all windows local administrators will loose admin privileges to the analysis services server. However, if you have explicitly added a user to the analysis services built-in Administrator role, those users will still continue to have admin rights.