The Leader of the Opposition of Singapore is usually the leader of the second largest political party represented in the Parliament of Singapore. During the 1955 Legislative Assembly election, the late Lee Kuan Yew was the "de facto" Leader of the Opposition, as the People's Action Party was then the second largest political party represented in the Legislative Assembly.
During Singapore's 1958 Legislative Assembly election, the People's Action Party was the second largest political party represented in the Legislative Assembly. 
The context states that in the 1955 Legislative Assembly election, the People's Action Party was the second largest political party represented in the Legislative Assembly. It does not mention 1958, so we don't know if this statement would have been true or not. The system was likely confused because it assumed that the statement's inclusion of a different date from the one in the context meant that the statement was wrong