coordinates - Importing an object from Blender into a scene, rotation on X axis?

This is my situation:

I save the scene with blender no export with any processing steps.

Blender has x right y up -z into the scene for the view coordinates (OpenGL) I have x right y up -z into the scene for the view coordinates (OpenGl) Bleneder has x/y plane and z up as world coordinates I have x/y plane and z up as world coordinates

I load the mesh with assimp directly from the blend file with absolutely no post processing. The object is rotated abount π/2 on the x-axis. Why?