App that retains data between installations. iCloud Keychain has never been enabled
iOS apps can keep important data (like login credentials) in the local Keychain. It is not the same keychain as the iCloud Keychain. iCloud Keychain is located in iCloud. Local keychain is located on your device, will never leave your device, only when you backup your iOS device and only when those backups are encrypted.

When you delete an app from your iOS device, there isn't really a uninstall procedure that also deletes keychain data. 

What you could try is to sign off in Pinterest before you delete the app. If the Pinterest app is developed properly the sign off function should delete the login credentials. (not sure about it)