Why are Brazil’s presidential polls so different from one another?

At the beginning of June, a presidential poll by DataPoder360 – a company created by news website Poder360 –  created havoc in financial markets. It showed the election front-runners to be two candidates dreaded by investors: the far-right Jair Bolsonaro, who had a comfortable lead with 25 percent of voting intentions, and the center-left Ciro Gomes, with 12 percent. São Paulo’s stock market crashed by 6 percent, and the Brazilian Real lost 2 percent against the U.S. Dollar, which closed June 7 at BRL 3.95. 

Three days later, Datafolha – Brazil’s most renowned polling institute – published its own poll, with different numbers. While Mr. Bolsonaro was also leading in all scenarios that exclude former President Luiz Inácio Lula da Silva (currently in jail and unlikely to be able to run for a third term), his numbers were much lower, around 19 percent. 

A third institute, Ipespe, was hired by an investment bank to hold presidential polls which targeted investors – and threw up another set of results. In scenarios where Lula is considered as a candidate, Mr. Bolsonaro narrowed the gap – and the poll even places him in a better position than...