When William I conquered England he allocated the land to his followers in small parcels. So Knight/Lord A would get 2 parcels of land in Sussex, 1 in Dorset, 1 in Lincolnshire, 2 in Lancashire and so on. This was to ensure that an individual could not establish a land base in one area and rebel. This of course did not stop a number of them joining forces and doing the same thing.
This meant that Knight/Lord A in the 10th century might decide to build his main home/castle in an area he liked. later on he might be awarded a peerage but that would be based a county that was available.
On a related subject you might think that not all counties became peerages. Some of the took the name of the chief city:
Arundel - Sussex
Salisbury - Wiltshire
etc.
They never existed at the same time.