Password for this site

I made a account and im wondered about the following:

I have a 16 digits/numbers password.

No words, no birthdays, etc.
I use letters and numbers and its random as can be.

Why does your password checker says its "medium" safe????

I have 5 passwords this way in my head and use them according to importance.
So the pass im using here is used also for "same level" logins elsewhere.

I always had 100% safe pass. Why is your site telling me it can be better?

Something like: (following isnt a real password its just a example)

isardaiwtpib9835

should be safe...

(in case you are wondering how to remember the above pass: "I See A Red Door And I Want To Paint It Black - a old login code from work or locker or..."