Differing Hole Notations In Eagle CAD?
As specified in the EAGLE manual on page 292, this is intentional. EAGLE uses 19 different drill symbols. The 18 symbols shown below are used for drills of a specified diameter. You can specify which symbol to use for which drill size in the Layout Editor under Options -> Set -> Drill. 

 (image from EAGLE manual)

Finally, the symbol Ø is used for drill sizes, which were not specified in the settings.

This allows you to easily recognize the different drill sizes. An additional drill legend can be generated with the ULP script drill­legend.ulp.