batch window lower than 20s but lambda function is called in 20s interval

In lambda manual, there is:

If you're using a batch window and your SQS queue contains very low traffic, Lambda might wait for up to 20 seconds before invoking your function. This is true even if you set a batch window lower than 20 seconds.

What is the cause behind this? I asked chatGPT and Bard, but neither of them could give me a solid answer.