I've found very little information on this topic after much googling. The only partial answer I found was by Microsoft: Microsoft_AES_Technical_Factsheet
By adding random data to each message, each message is unique even if the same letters are typed over and over. This prevents frequency analysis from finding identical messages to track.
which makes sense but leaves me wanting to know:
A) How random data can be added to the messages without causing noise in the signal, and
B) How other manufacturers prevent frequency analysis. I could find nothing regarding Logitech, for example.
Is it simply an industry standard to 'just add noise', therefore it can be safely assumed to be implemented?