DRC errors in Encounter using custom std cell library

Hello,

I  have tried a simple place&route with encounter without any timing optimizations.I simply placed the cells.I am using a custom std cell library in which the cells are very dense and there is significant blockage in ME1.
 I also have connections with higher metals in the std.cells. What i find weird it that after the routing there are DRC violations because Nanoroute used ME1 in the regions where there is blockage in ME1, even thought there is no blockage in ME2 and ME3 and it could very easily have no problems using them.

I would like to ask now, If someone could give me a short explanation on how the router chooses the metal that is going to use with respect to the blockages,pins (location and layer) .Could the lef of the technology be involved in this behavior?Could i control the layers used for specific connections apart from changing them manually?

Thank you in advance!