She shouldn't have lied but maybe it was just an instant reaction without really thinking before she had chance to respond? I'm sure we've all done that before and I can't really think of any other instance where she's been dishonest?
However Emma is a complete liar, she's admitted that she didn't nominate Dean for the reasons she gave and she even told Rosie she didn't hear any boos for her then said after she did when Rosie told her not to lie.

Adding to the fact of the amount of times she's bitched about Lily then acted like they were best friends. She's so awful and two-faced and insanely desperate to win