I'm not at a workstation to check, but I recall it having to do with either:
- Pipe drafting direction (drafting from fixture to main is consistently correct for me)
- Pipe system flow direction, which you would not currently have established in the photo as there are three connectors open
EDIT: Gave it a go. When you draft from the main 'up' into the fixture, the trap position winds up being on the wrong plane like in your example. If you are using plumbing fixture families or any family with pipe connectors, using the Connect Into command, clicking the connector graphic to start the Draw Pipe command, or right-clicking the connector and choosing Draw Pipe will all guarantee that the drafting direction (I need a better term for this that is more modelling-appropriate) is correct for p-trap alignment.
And for what it is worth, the P-trap family I'm using is the OOTB one, with Part Type set to Elbow.