‘For as touching the ministering to the saints, it is superfluous for me to write to you, for I know your readiness, of which I glory on your behalf to those of Macedonia, that Achaia has been prepared for a year past, and your zeal has stirred up very many of them.'
Note the connecting ‘for' which connects the chapter with his words in chapter 2 Corinthians 8:24. It reads as though it is as a result of his mentioning of his glorying in the Corinthians in the previous verse that he now writes as he does.
This verse must surely be seen as a piece of delicate diplomacy. Having urged on them incentives for them to make their gifts, including the self-sacrificing giving of the Macedonians, he now back-pedals a little and assures them that he realises that what he has said was in fact superfluous, and need not have been said, because he does indeed know of their present readiness to collect funds for Jerusalem, and has already boasted about it to the Macedonians. The fact of the matter is that he had not only informed the Macedonians that Achaia has already been collecting funds and had been ready for a whole year to contribute towards the collection, but that he had actually done this to such an extent that their zeal had aroused others to give.
It may also be that he is bearing in mind that he is speaking in his letter to two audiences. The main church in Corinth, with whom he had been at cross purposes, who may have slackened their zeal for the Collection, and the other churches in the area known locally as Achaia, around Corinth, who may not have been involved and may have therefore have continued collecting apace. And he would know that his letter would be read in both places, with the right emphasis being passed to each by the bearer. (That is why such a pastoral letter can sometimes appear to be saying two slightly different things. And he would certainly not be the only writer to repeat himself from a slightly different viewpoint when he has a point that he desperately wants to get over).