If the average difference between the figures for compound interest and simple interest is say, €3,000, then AIB could save in the region of €18m by applying simple interest.
I think they are trying to pull a fast one and didn’t bank on so many payees being on this forum and therefore being in a position to call them out.
Even their explanation of how they calculated the interest in the Q&A document is convoluted - using terms like “static” etc instead of simply stating “we used a simple interest calculation on the 12% to calculate the amount”.
AIB are, I believe, chancing it and in so doing, pushing the bounds of credibility.