Why does arcpy.da.Walk work differently when used with file geodatabase (gdb) and personal geodatabase (mdb)?

Question

Why does arcpy.da.Walk give different output with identical File (GDB) and Personal (MDB) Geodatabases?

Example:

I am using ArcMap 10.4.1

r'D:\SCRipt\New.mdb' is a copy of r'D:\SCRipt\New.gdb'

---

db = r'D:\SCRipt\New.mdb'
    walk = arcpy.da.Walk(db)
    for x,y,z in walk:
        print x
        print y
        print z

produces this output:

D:\SCRipt\New.mdb
[u'FORESTS']
[u'Layout10', u'Layout1']
D:\SCRipt\New.mdb\FORESTS
[]
[u'Vydel', u'Kvartal', u'Vydel_S', u'Vydel_L', u'Lesnich', u'Leshoz', u'Layout1_Vydel']

---

db = r'D:\SCRipt\New.gdb'
walk = arcpy.da.Walk(db)
    for x,y,z in walk:
        print x
        print y
        print z

produces this different output:

D:\SCRipt\New.gdb
[u'FORESTS']
[u'Layout1', u'Layout10']
D:\SCRipt\New.gdb\FORESTS
[]
[] ## <- Why?