What is the reason for different recording times for different channels of the same conversation in Asterisk?

Question

Asterisk

Greetings!
A strange problem has arisen. There is a SIP peer, the call is made through a simple Dial without MOH, etc. Recording of conversations is enabled, the command is
MixMonitor(${filepath},b r(${afile})t(${bfile}))
as follows: And it turns out that channel b is shorter in duration of channel a by ~ 10 seconds, and the mixed file does not contain any shifts. What can cause such behavior?
Asterisk 14.6.2
Thanks!