why does my iphone show SAMSUNG SGH-i677

That's the name of the particular contact you have set as your card.

The name is whatever you set in that contact. It has nothing to do with your iPhone.

Find the contact and edit it as required so it shows whatever name you want, or set a different contact as your Card.

click here ➜ Add your contact info on iPhone - Apple Support

Because it was set in that contact as its phone number.

The contact is not automatically updated.

If you had that number when you created that contact and set that number for it, then that's the number the contact keeps unless you manually update it.