Hugh WILLOUGHBY - Unlikely Parentage from Dates

This profile has date of birth recorded as 1380 and some of the attached matches, for what they are worth have 1393.

Hugh's mother Isabelle Isabel Willoughby however has date of birth recorded as c1383 which are just about consistent with the dates of birth of her own parents.

I'm afraid I dont have the knowledge of how to decipher the family tree pictured in the attached link in the Overview to Thoroton's Nottinghamshire https://books.google.com.au/books?id=-teYO3hFqE0C&newbks=0&... to determine what the documented dates should be.

However, it is clear that something is wrong and the the parentage, at least of Hugh's mother is highly suspicious.

Does anyone have the knowledge to resolve this?