Rounding error

I'm not sure whether this is me or a bug. 

The round component is giving me different results, when identical information is fed in. 

I have isolated down to just two, and cannot see any reason why. I would guess its to do with tolerance, but i'm not sure how to increase accuracy.

At the moment i am bypassing the problem by adding 0.000001 to the number before rounding down. But would like to know why

Thanks