Family History

So I'm currently browsing through the 1940 US Census data that was released last week, and came upon some interesting information! I looked up my paternal grandfather, who lived in Fitchburg, Massachusetts, and was able to find his family's information. Now, as I understood it, he was raised with 2 brothers and his mom and dad. However, when I looked up the information, his father and mother had 8 kids listed, and my grandfather's two brothers were not listed as children! Now I can't remember, if my grandfather was the youngest of his brothers or not. He's listed as being 17 years old on the census form (which is slightly inaccurate - he was born in 1921, which would've made him 18 or 19), and if his brothers were both older, they would've probably left the house by then. I'm really curious who these other 8 people are listed under my great-grandfather's name! I'm definitely going to ask my dad tomorrow and see if he knows anything about this!