There's another discussion here, with answer: https://forums.elderscrollsonline.com/en/discussion/comment/6788134#Comment_6788134

Long and short was it was never supposed to be guaranteed.

I gotta say, they gained a lot of goodwill from me by making it guaranteed. It shows they weren't completely devoted to the grindy RNG-based nonsense that, as people who value good game design, they ought to resist doing too much of.

Learning that the guarantee was a mistake made all that goodwill and more go away.

(Which isn't to say there should be NO RNG-based grinds, just that an over-reliance is something good designers would would avoid.)