Totally guessing here but it seems the basic problem was the OP interpreted (if seen at all) the road marking in OP's lane on the approach as indicating OP could continue left into the straight. This is not a difficult interpretation to understand and anyone could make the assumption at this point of the road.
The road marking in the other lane (http://goo.gl/maps/WFkoG) if seen does give a better idea of the structure of the upcoming roadway. But this is easy to miss or obscured by something like a bus.
Someone has noticed the number of accidents here and erected the green sign (http://goo.gl/maps/mlxzRin an attempt to provide more information and reduce the accident rate. The green sign is informational only and intended to give warning about the layout of the road ahead.
Really a red/black regulatory sign is needed here directing what is and is not allowed. It would get more attention than an information sign and semantically define the correct behavior. My guess is there is no standard red/black regulatory sign for this purpose with wiggly lines on it so a more flexible green one is used as a compromise.
Another OP factor may be experience coming from the other direction and assuming the rules are consistent from each direction. OP may have believed the intersection behaved the same from both approaches and expected the bus to head west like it would from the other direction.
It's interesting looking at it closer to the turn: http://goo.gl/maps/K7dko. If there are lanes here they are marked straight ahead. So again - it is easy to see how a belief could have continued. The OP is expecting the bus to go straight by following the marked lines.
If you were looking at that picture without prior knowledge of the intersection you would say the bus must wait on the left until the OP has passed before making the turn right and across the right hand lane. In reality you would not expect the bus to make that turn at all from the picture.
Maybe I don't drive often enough - guess what - 10 minutes in the car today - complete change of mind from before!

I hope this turns out well for everyone.