worn tyre?

skoda 1.6 tdi cr.

this may be a difficult question to answer without photos so I will explain as best I can.

both front tyres are evenly worn but the o/s/f tyre has developed a raised ridge on the outside edge/shoulder of the tread.
steering is nice and straight, no vibes, no rattles/knocks, no wandering/pulling to one side.
the tyres do need replacing as they are down to the limit. has the tyre worn like this because of little tread as I only noticed it like this couple of weeks ago. I do inspect/service my car on a regular basis. any ideas. thanks