The Default behavior is Immediate, which is what old games before the change used. After the change, confusing as it is, the default property is actually Deferred, not Default. If you would like to read more on deferred behavior, read this post and any information linked: Deferred Engine Events
This affects when your connections gets called. In this case, Immediate mode will trigger your CharacterAdded code without your character even parented in the workspace yet, and internal code to function your character is not done. With Deferred mode, it will trigger at the next cycle and your code will basically run when your character is already in the workspace and the internal code is (almost always) done, avoiding race conditions.