Part Prepayment - how does the tenure always decrease in whole numbers?

Whenever I do part prepayment of my home loan, I always ask them to reduce the tenure and keep the installment same. I part-prepay anywhere between 1 to 6 EMIs (that's the maximum allowed per month without visiting the branch).

I am curious as how always the tenure reduces by a whole number of months. i.e. if I pay pre-pay 6 EMIs, my tenure reduces by say 10 months. I have never had the tenure reduce by 9.5 months or 2.75 months etc.

The only reason how I think this would be possible is if that the last month in the reduced tenure would be not be 1 whole EMI but would be a part EMI.