You bought in May, & you would have been entitled to relief for the entire year (Jan – Dec), so your relief for the entire year would have been spread out over 6 months instead of 12 months last year, having the effect of increasing your monthly TRS for May – Dec.
Your TRS should have gone back down to the normal monthly amount in January 2010, so I don’t know why it only reduced this month – it may be that the bank didn’t adjust it on their system, or that they waited for Revenue to tell them to.
For your own peace of mind you should ring Revenue TRS section (1890 463 626) & get them to confirm this.