In line series,
mouseover is triggered when the mouse pointer is on a data point. This should only happen when the mouseover event is in the dataPoints object.
If the mouseover event is written in the series object then it should trigger when the mouse is anywhere on the line.
Check this fiddle,
there is no difference in the two mouseover events, both are fired when the mouse is over the point. The outer one should trigger also when the mouse is on the line.