Discrepancy between particle trajectory and streamline?

Hello,

I'm trying to teach myself how to use the particle tracing module, so I'm modeling a simple T-junction microfluidic channel. I get an unexpected result, however, when I trace the particles. Since it is a microchannel, I expect the particles to follow streamlines. However, they do not (see attached). I've tried both the Newtonian and massless configurations. This is a 2D, incompressible model, assuming a shallow channel of 30 um. Could anyone lend any insight into what's happening or which results I should trust?

Thanks,
Alisha