Why Scheer used separate shore and sea call signs

Why did Room 40 have two call signs for the same German commander?

Scheer used one ashore and another at sea to confuse British interception, but Room 40 already understood the distinction.

Room 40's two identifiers reflected Scheer's own use of different call signs in harbour and at sea. The German arrangement was intended to confuse British signals intelligence. Room 40 had already penetrated German communications far enough to recognize it. Following one call sign therefore required knowing whether it travelled with the commander.