Its likely with how you're parsing what's legitimately a real page, what is a 404 and what happens when a site confuses the two.
www.whoismrrobot.com, for example, doesn't have a 404 - it redirects anything that would be a 404 back to the main page, so you get this as a result:
=====================================================
Gobuster v0.8 (DIR support by OJ Reeves @TheColonial)
(DNS support by Peleus @0x42424242)
=====================================================
[+] Mode : dir
[+] Url/Domain : http://www.whoismrrobot.com/
[+] Threads : 10
[+] Wordlist : list.txt
[+] Status codes : 200,204,301,302,307
=====================================================
/A (200)
/about-us (200)
/aboutus (200)
/acceso (200)
/academics (200)
/abstract (200)
/aboutUs (200)
/about_us (200)
/AboutUs (200)
/About (200)
/accesswatch (200)
/accessories (200)
/action (200)
/accounts (200)
/acciones (200)
/activities (200)
/active (200)
/accessibility (200)
/ad (200)
/adclick (200)
/adlog (200)
/admcgi (200)
/admin-bak (200)
/adm (200)
/admin-old (200)
/admin-console (200)
/admin.back (200)
/add (200)```
Its likely with how you're parsing what's legitimately a real page, what is a 404 and what happens when a site confuses the two.
www.whoismrrobot.com, for example, doesn't have a 404 - it redirects anything that would be a 404 back to the main page, so you get this as a result: