Fair enough. I will take that at face value.
First, the Hindi spoken in western UP is almost incomprehensible to the Bihari or to the person from eastern UP (the Bihari and the eastern UP guys can understand each other very well - traditionally, they speak of A-B-C Arrah, Balliah and Chhapra).
The culture is most dissimilar. They don't get along for that reason. There are separate festivals and gatherings of the eastern folk, the Purbiyas, on most festive occasions which are shared occasions among Hindus of all kinds. Right down to the difference between the salwar-kameez wearing western UP-ites and the sari-wearing easterners, through the food, through the marriage customs and specifics.
Their religious habits are vastly different. I am keeping this short, almost telegraphic, for personal reasons; I have to get off the system and take my medicines, which are already two hours overdue. Apologies for the brevity.
Finally, western UP is Jat and Gujjar dominated. Bihar has a cocktail of people who are entirely different and even look different.
I know that this is a series of bald assertions, but that is the best I can do about it for today. If you care to remind me ten days later, I could walk you through some of these differences.