From what I think I know... If all students are engaged and making excellent progress then it is much easier to get a Outstanding. Maybe you aren't doing anything different but maybe the children know you better and you feel more comfortable. Little things like tone of voice and control of the class can give an impression of success, both to the observer and the students, that you may not be aware of. I think you should be proud that you are scoring an outstanding and spend some time looking at the Ofsted criteria yourself. Use a highlighter, go through what you think you are doing. Ask for some observation CPD with your mentor. This way, you can go through things together. Lots of observers WON'T have been Ofsted trained, or so I've heard. Then again, I'm only an NQT myself!