I just tested build 1579 and compared the two and there are some quite obvious issues when used on older versions of Windows or if themes are disabled.
In build 1579 the borders aren't being applied correctly.
Modern unchecked (Windows 7)
Modern checked (Windows 7)
Modern unchecked (Windows XP)
Modern checked (Windows XP)
This fix will be applied to the next release.
The problem was caused by extra space being added for the edge borders when there were no borders on the toolbar.