Many factors could lead to this. Rural urban migration for example. An informed position would be to compare the number of births and deaths recorded in the 10 years. Note Census data is not reflective of voter numbers as i've seen uninformed media yappers saying. People leave cities and towns to vote for their candidates unfortunately or is it fortunately the monies disbursed for counties and cdf is based on census data.

Big question should be what constituencies will cease to exist now?

Are the borders of Nyamira in 2009 same as 2019?

There were no counties in 2009. As much as the census in their backdated tally would fall back current constituencies into 2009 'counties' the constituencies borders were also redrawn with 2010 Constitution.

Where is this magic number defined?

But many many constituencies will go if that number is adhered to!!

Let me speak for Nyanza, the 7 districts that existed before 2010 were just converted into Counties with the boundaries remaining intact. So Nyamira District then is Nyamira county now and thereby the ease of comparison.

Hao wz Isiolo wengi ni wameru who have moved to Isiolo to make money. Plus the leaders there were very persistent and led a drive to ensure that even nomads were counted.