Symbology class doesn't show without scale restriction?

[ChristopherBertrand]

I have a roads feature class that has been symbolized by type; Interstates, State Highways, frontage roads, and other (Roads).  It works fine but for one thing. When the scale isn't restricted for the roads class (other values) it doesn't display.  I expect if I removed, reloaded, and re-symbolized the layer this wouldn't happen but I would like to know why it is so it doesn't happen again.  Does anyone have any insight into possible causes?