I can think of at least four major events that wouldn't have been predicted that would send the figures out:
1) World War II and the Holocaust. It was only in September 1930, that Hitler/Nazis got 18% of the vote in the Reichstage elections. Before then, they were politically irrelevant, so that wouldn't have been predicted.
2) The Soviet occupation of Eastern Europe and the fall of the Iron Curtian (1945-48) would have had an affect on their religious populations.Even if you tried to calculate religious populations in the USSR in 1930 when it was had a "five year plan" to eliminate religion, the effects of world war II (somewhere between 20 and 40 million deaths) would be hard to calculate.
3) In 1930, China was in the midst of a brutal civil war, and in 1931, the Japanese invaded and set up the puppet state of Manchquo. The Second Sino-Japanese War (1937-1945) killed between 20 and 35 million people. The People's Republic of China was founded in 1949 with a population of 540 million or so, about 1/5 of the world's population for 1950. 1970 falls slap bang in the middle of the Cultural Revolution period (1966-1976) so that would send the statistics for many religions out or make it hard to estimate. The Great Leap Forward (1959-61) also had a major effect on world population statistics as the largest famine in history with estimates ranging from 18 to 35 million people.
4) the Partition of India (1947) would have affected statistics as the region had a very large population (Pakistan and India probably equal to China). 200,000 -500,000 people died in religious motivated genocides, and 14 million people were displaced.
Just reeling those figures out is a little bit shocking for how destructive that period was.
There's also the "What ifs" of the Cuban Missile Crisis in 1962 in that period as well, but I think any futurologist would want to leave that one alone. In 1930 people were worried about the effects of ariel warfare attacking civilians. nuclear warfare was still something yet to be imagined. it would be an act of cruelty for a time traveller to finish the story in October 1962 without saying how that 'collective nervous breakdown' ended.
Yeah. 1930 was not a good year to look into the future.