Jessica likes both, but prefers chocolate; she just ate vanilla the previous time because she had a craving for it.
Jessica always, without fail, chooses chocolate over vanilla - but she will eat vanilla, if chocolate is not available.
It's pointed out, only minorly, that Jessica eating vanilla is rather strange since she could choose chocolate. This only became more pronounced, but not to the point of flanderization, later, because of an arc that involved going to get something to eat.
Jessica was a well-developed character when introduced, but writers have taken to making her do whatever the plot requires; most noticably, she liked chocolate, but now can absolutely insist on having vanilla one episode, strawberry the next, peppermint the third, and each time she says it's the only flavor she has ever liked.
Subverted: Jessica loves chocolate, but she loves vanilla even more.
Double Subverted: ...but she prefers to save vanilla for special occasions.
Parodied: Another character talks to Jessica about when she liked vanilla, but Jessica has no memory of it.