Why German movements looked like an Atlantic breakout

Why did the Royal Navy expect a German Atlantic breakout instead of an invasion of Norway?

The reported force looked like a breakout group, and planners assumed Norway would mobilize effectively enough to make invasion irrational.

British planners judged that Germany's small navy would be taking an extraordinary risk by invading Norway. They expected capable Norwegian forces to make landing troops from warships very dangerous. The answer faults the Norwegian government's slow mobilization by telegram and post, arguing that an open radio call could have brought defenders into position sooner. Fragmentary intelligence reaching the Admiralty and Home Fleet described capital ships, possible raiders, and destroyers heading north, with no transports identified. Earlier German movements through northern waters reinforced the breakout interpretation. The strong escort could be understood as a screen intended to force raiders through the British patrol line.