A source associates with one only page. Why would Google Analytics report more than one landing page?

In my understanding, in Google Analytics, a landing page is the first page the user visit in a session. If for a specific UTM parameter (e.g. source) there is only one page using it, then in the Landing Page report, that’s the only page associating with the parameter.

However, it doesn’t seem so in my report. I’m sure that the source I use only using one page.

Do you know why?