Why does the noise need to be increased during encryption when the degree of the cyclotomic polynomial is not a power of two?

[sysuWenchao]

Why is it necessary to increase the standard deviation (stdev) of the noise during the encryption process when the degree m of the cyclotomic polynomial is not a power of two? For example, in the code snippet:

if (context.getZMStar().getPow2() == 0) // not power of two
    stdev *= sqrt(context.getM());

https://github.com/homenc/HElib/blob/master/src/keys.cpp

I would greatly appreciate it if you could provide some insights. Thank you for your guidance!