fragmented index not defragmenting after an index rebuild
Yes I see that fragmentation all the time with GUID columns, due to the random inserts and page splits. In your case, this could be a result of a non-standard fill factor to account for GUID fields.

A clustering key should be on an ever-increasing, narrow data type, like INT or BIGINT, and the fill factor set as close to 100 (or 0) as possible.

That all said, rebuilding a clustered index due to fragmentation is a time-consuming process, and it generates a lot of transaction log, so it's generally more accepted to simply update the statistics fairly frequently.

You can worry about fragmentation less frequently, say once a month.