when i use the erase tool i draw a line, when i finish drawing the line and release the mouse button the red highlighted mask is different that what I draw

I am trying to erase  a  line using the eraser tool 

I draw a line  when complete the  the line and release the mouse button the highlighted mask changes and shows other areas 

In the picture I draw  one stroke form the start of  the white trail to the and after i release the button the mask changes in to the below result . the problem is constantly repeated 

After that I tried some random strokes in different places  I get always  the left bottom highlight