telegraf gives different results when testing

Question

Running telegraf 1.24.0

telegraf -config ./telegraf.conf -test -once 1>output 2>err

In my telegraf.conf I have

[[outputs.file]]
files=["stdout"] #staticfile

[[inputs.tail]]
files=["syslog"]
from_beginning=true
data_format="grok"
grok_patterns=['%{SYSLOGTIMESTAMP:timestamp} %{GREEDYDATA:message}']

when I run

for i in $(seq 1 10); do 
    telegraf -config ./telegraf.conf -test -once 1>output 2>err
    grep -c ^tail, output
  done

I get different values for tail. Why is that? The "syslog" file is static, its not moving, so I am not sure why the values keep changing when running telegraf