WTC - relevant years...?

I thought that I understood WTC but looking through my renewal form, I'm not sure if I do. My question relates to the relevant years.

I claimed WTC for the first time in 2012-13 based on an estimate of my income in 11-12. Now, in my renewal form they are asking me to confirm my income for 11-12 and estimate my income for 12-13. That implies to me that the WTC I was entitled to for 12-13 is based on my final income for 11-12. Is that correct?

Or, is my entitlement to WTC 12-13 actually based, in the end, on my 12-13 income? That would seem more logical but doesn't actually explain why I am having to confirm my 11-12 income when I did not claim at all in that time period.

Thanks!