What does churchill mean when he described russia as being “ a riddle, wrapped in a mystery, inside an enigma”?

This is how Churchill expressed inability to predict what Soviet Union will at the start of World War 2. The timeline below show why he could be puzzled:

a) August 23,  1939 - Soviet Union and Germany sign non-aggression pact

b) September 1, 1939 - Germany invades Poland (then ally of Britain and France).

c) September 3, 1939 - Britain declares war on Germany as a consequence of German's invasion of Poland.

d) September 17, 1939 - Soviet Union invades eastern Poland. Poland is defeated and partitioned between Germany and Soviet Union.

Therefore from Churchill standpoint: Soviet Union is allied with Germany, whom are at war with Britain. Go figure what Stalin is going to do?

our current president donald trump