I've gotten those stripes plenty of times while filing blades back in the day! Usually it occurred only towards the ricasso, and it has something to do with the blade vibrating slightly as the file is drawn across it, as it is being held down by clamps on the tang. I couldn't always get rid of that weird effect, usually I would change how I had the tang clamped or change what I had the tang clamped onto. The file does not bite as efficiently as it should when this vibration begins to happen. Would drive me bananas when it happened.