Why the "latency.msec" is not equals to input's "timestamp.latency" plus output's "timestamp.latency" in "Round Trip Latency"

Android version(s): 11.0
Android device(s): HUAWEI mate 30 pro
Oboe version: 1.5.1
OboeTester version 1.6.3
App name used for testing: OboeTester

**Short description**
Why the "latency.mesc" is not equals to input's "timestamp.latency" plus output's "timestamp.latency" in "Round Trip Latency"

**Steps to reproduce**
Just At "Round Trip Latency" Page in OboeTester App

message like below:
INPUT: timestamp.latency = 8.5/11.0/13.4ms
OUTPUT timestamp.latency = 3.1/5.5/7.9ms
MEASURE RESULT: latency.msec = 42.65, confidence = 0.613

abviously, 13.4 + 7.9 = 21.3 != 42.65
And repeat multiple times, the result is not equal, and there has a big difference between them.