Prometheus increase result problem

hi everyone,
When I use increase function to calculate the increment, I expected an integer, but a value with a decimal appeared instead.

First, I check the value was 913028805.
Then I made the value up to 913029005, it should be 200 when I use increase{…[1m]} to calculate the value but it appeared to be 266.666.

when I use increase{…[5m]}, it become close to 200.
The source data appears to be fine.

Here is my Prometheus config.yml

  scrape_interval: 15s # Set the scrape interval to every 15 seconds. Default is every 1 minute.
  evaluation_interval: 15s # Evaluate rules every 15 seconds. The default is every 1 minute.
  # scrape_timeout is set to the global default (10s).

# Alertmanager configuration
    - static_configs:
        - targets: ['']