`type -a` works even though `-a` not an option I could check installed apps with command type -a

$ type -a python
python is /Users/gaowei/Desktop/Django/learning_log/ll_env/bin/python
python is /Users/gaowei/anaconda3/bin/python
python is /usr/bin/python


However, there's no -a options in 'man type'

What's the reason?
If you type help type then you will see that type does have the -a option.