On some NAT routes on the OTS, not all levels are utilized and will be omitted from Track Message. For instance, during daytime, on the westbound flow, you may have Track "Charlie" with available levels 310,320,330,340,360,370,390. So, FL 350 and 380 are available for use by flights on a random routing, which could be opposite the westbound flow. That is why you see opposite direction traffic.