Yandex Tank: threads not const and not same RPS

Question

We have is very simple Web service. HTTP/HTTPS request on out service from Internet clients. In HEAD return UID.

We want to testing our service by Yandex Tank.

load.yml

phantom:
  address: 211.81.41.11:443 #IP тестового стенда, порт 443
#  address: her.your.ru:443 #IP тестового стенда, порт 443
  ssl: true #use https
  uris:
    - "/"
  load_profile:
    load_type: rps
    schedule: const(5000, 320s) удержание 5000 rps в течение 320 сек
  instances: 5000
  header_http: "1.1"
  headers:
    - "[Host: her.your.ru]"
    - "[Connection: keep-alive]"
    - "[User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36]"
uploader: #модуль для постройки графика
  enabled: true
  operator: my-username
  package: yandextank.plugins.DataUploader
  token_file: token.txt
console:
  enabled: true
telegraf:
  enabled: false

start

docker run --rm -v /opt/docker/yandex.tank/her.your.ru:/var/loadtest -it direvius/yandex-tank

testing is done

check and saw this problem: threads not const and not same RPS.

Why exist this problem? what check on host where I run testing (4CPU, 16Gb: during testing utilization CPUs less 20%) and how solve this problem?