User blog comment:Miskos3/Fairy Tail 525: Why Did the Emperor Never Love His Child?, Review/@comment-25111584-20170304133617/@comment-7418318-20170304170455

Considering Mavis nature and intelligence it's entirely possible that rather than talking out of her heart when she told the others to believe in Natsu, she just said what she judged to be the most likely thing to get them moving.

While she did show this naive overly trusting side on occasion, she's also a master strategist (on occasion...) and that comes with a frote to manipulate others into doing what you want them to do.