Brochas Daf Chof Zayin amud beis

There was an incident involving a certain disciple who came before R. Yehoshua. He said to him, is the evening prayer elective or obligatory? He replied it is elective...

...The questioner arose (in the study hall) and asked,is the evening prayer elective or obligatory? Raban Gamliel replied it is compulsory.  Raban Gamliel said to the sages, is there anyone here who disputes this ruling. R. Yehoshua said to him, "no."

The Ben Yehoyada asks how R. Yehoshua could change his response to the same question. He responds that the person asking the question was different and thus appropriately elicited different answers.

The disciple cited in the first part of the Brysa was, as the Gemara tells us, R. Yochanan ben Zachai. The questioner in the second half of the Brysa was the "Torgoman," the official designated within the study hall to announce the questions.

When R. Yehoshua was asked a question by R. Yochanan ben Zachai he realised that the question was not designed to elicit the halacha because once Jews "took on" the obligations of Maariv it became obligatory for them. Rather the question was designed to elicit either the essence of Maariv or alternatively to determine the relative priorities associated with fulfilling some other mitzva at the time Maariv falls due.

In the Beis Medrash when the questioner was the Torgoman, R. Yehoshua had to assume he was being asked about the basic Halacha, and therefore had no doubt in responding that it was obligatory.