You would think that Manazane would be a great item for Syndra but it has a positive Delta (positive Delta means that Syndra is worse off for equipping that item compared to others).

This is because Syndra has a long cast time animation. Normally, a unit buffers the Manazane mana for the next case, but Syndra’s mana pool is so low she doesn’t benefit much from the buffer.

Thank you to Ramblinnn for the explanation.