The Norman Feudal System and Walter de Lacy

1066 and the battle of Hastings. Walter Lacy fought under the command of William Fitz Osbern Lord of Breteuil. Why?

His brother Ilbert de Lacy fought under the family Overlord Bishop Odo of Bayeux who in turn was a Vassal to his half-brother the Duke of Normandy. The Lacy family were vassals of Bishop Odo under the Norman feudal system.

Within the next few years, William Fitz Osbern now the Earl of Hereford and Vassal to his half-brother the Duke of Normandy conceded much of his Hereford lands to Walter de Lacy and his Son Roger. At the same time, King William made Walter the first Lord of Weobley.

What had led to Walter de Lacy and his son Roger becoming a vassal of William Fitz Osbern while the rest of the de Lacy Family remained under Bishop Odo as their overlord? Ilbert and Walter had retained close family ties only time and distance separated the descendants of the two family branches.

From Lords down to Pheasants every family had to swear before God an oath of fidelity. In an uneducated God-fearing country few of any rank had the courage to try and break their oath.

Once the feudal system was in operation throughout Norman England. King William was able to sophisticate the gathering of organised taxes by creating the Doomsday Book. Here was listed all essential possessions throughout the land. It was down to each following King or overlord as to whether this was a fair system.

I have added a link to an amusing video which explains just what the Feudal system was and how it worked.

So I return to my original question, why were the two de Lacy brothers fighting under different overlords? ​Do we have anyone with historical connections who may shine some light on the question?