I'm sure this has already been mentioned, but she would've seen Jason in his swimsuit while he was living at the mansion.
Speaking of which... something pretty magical must've happened during that day alone with Deanna for Jason to have had that blissful expression on his face when greeting the other guys. If I didn't like him so much, I might just call it 'cocky', but I do so I'll just call him 'very confident'.
The fact that they both woke up and, I'm assuming, had breakfast, lunch, and dinner together definitely gave him an 'edge'-- no doubt about it. Add in the fact that they could've hung out at the pool, used the jacuzzi--he had his first 'fantasy date' 2 weeks before the other guys.