Hello,
This is to be expected. While the site receives constant traffic, the scheduling algorithm has only one or two minutes of delay. If your site does not receive traffic, the transactions will happen later, but they will go through because everything is designed to be asynchronous. If you need more accuracy you could setup an actual cron trigger which will force the site to run more often.
I have also made sure to take timezones into account. Can you check that you have set the correct timezone in WordPress?
with regards