The received pronunciation of 'centenary' seems for some reason to be changing from 'sen -TEEN- ary' to 'sen -TENN- ary.'
Another possible explanation is perhaps that there seems to be a general trend in the pronunciation of words which come from (or sound as if they come from) Latin as if they were still in Latin, rather than having become English. Three further examples noted today on Radio 4: 'supervizzary','eviskerate' and 'Lithooaynia'. (At least it's not 'Lithooahnia').