For me Carols explanation is by far the best.
If you label someone who has had a drink for the first time in about a year to have relapsed then that person might well think 'In for a penny, in for a pound'. In other words (just in case that is a UK saying) 'I might as well go on a 1 month bender if I am relapsed (failed) anyway'.
That said, if in the same circumstances someone had said to this person that they had, had a slip - but that it still constituted only 1 drinking session in 365 days (which is far more soberiety than even the 'normal' person enjoys) then it might well encourage that person to see that mistake for what it is. A one off mistake.
Maybe this is how I prefer it though? The preference is in the ear of the listener I guess? I would react in the way I have described above if I was told I had relapsed - but I would go straight back on the wagon if I was encouraged and told it was a slip.
Having said that it might work the opposite way round for some people and I accept that.