I experienced recently a defective (poor quality) new tyre which was initially round and symmetric, but bump it somewhere along the way and it just goes 'out of center', developing a spot where it even visually looks narrower. Tube was fine, it didn't have anything with it. I had to remount the tyre to make it round again, but for how long would it stand round before something similar occurs agains? So I just switched it to a better quality tyre. It was a cheap Bontrager 25x700C with wire. It's wire diameter was a bit too wide and I suspect this is what made something such possible.