IP Structure: Why HeaderLength is ahead of Version

Question

This is my ctypes code in Python:

class IP(Structure):
_fields_ = [
    ('ip_hl', c_ubyte, 4), 
    ('ip_version', c_ubyte, 4),
    ('tos', c_ubyte),
    ('len', c_ushort),
    ('id', c_ushort),
    ('offset', c_ushort),
    ('ttl', c_ubyte),
    ('protocol_num', c_ubyte),
    ('sum', c_ushort),
    ('src', c_ulong),
    ('dst', c_ulong)
]

As we all know in IP Header 0-3bit is Version and 4-7bit is HeaderLength but why in this code HeaderLength is ahead of Version