Mobs - an explenation please:

Ok, I notice a number of things I do not understand so I am here...

I notice mobs of the same level have 3 possible markings, from 1 to 3 strips.

I assume that reflect how well they fight, however, it seems that a level 10 (3 stripes) gives more experience than a level 11 (1 stripe).

In addition it seems that if a fight takes longer the xp reward is much higher.

Can somebody clarify this?

Thanks...