My understanding of it is becasue it is an interest first account. The idea is that you get your interest within the first month then are free to reinvest it.
Eg
You open an account with 10,000
3.95% of that is 395
Now you get that interest within one month. If you were able to lodge it to the same fixed term PTSB 1 year account (which you can't but you are free to invest it elsewhere) you'd get 3.95% *11/12*395 = 14.30
395 + 14.30/10,000 is about 4.1%.
I have one of these accounts and bought an An Post savings cert with the interest.
In the above example I'm not sure whether the "interest on the interest" should be multiplied by 11/12 or by 12/12. it doesn't make a big difference anyway.