Kerala State achieved replacement level fertility rate of 2.1 long before, but still have positive population growth

According to the SRS 2010, twenty two states/uts have achieved replacement level fertility of 2.1 and below. One may have a question that if twenty two states have achieved replacement level fertility of 2.1 and many are below 2.1 for quite a few years, then why these states are showing positive population growth.    

Kerala is having TFR much below 2.0 even through the last decade, but is depicting downward positive growth in population as given below:

Percentage Decadal Variation for Kerala State as per various Censuses conducted

1961-71	1971- 81	1981- 91	1991-01	2001-11
+26.29	+19.24	+14.32	+9.43	+4.86

The positive growth is due to population momentum factor.  Although the replacement level has been achieved, but the girls born much earlier would add one or more children when they get married and start producing their off springs.  This  population momentum occurs due to the past high birth rate when the couples born in the past will be in the reproductive ages in the coming years resulting in higher number of marriages and subsequent child birth which cannot be avoided by policies implemented henceforth.