You’ll find that for most of the items, you’ll see additional vocabulary pop up in the future that contains those items, which helps reinforce the readings. The readings differ between on’yomi and kun’yomi readings depending on what WaniKani feel is most useful to teach; based on frequently it’s seen, amount of vocab that use it, etc.
Also, while I’m here, and since it’s your first post…
\textcolor{pink}{\huge \textsf{WELCOME! ^-^}}
Take the time to check out the FAQ and GUIDE if you haven’t already.
There’s also a lot of good stuff on the forum to help you, like:
The Ultimate Guide for WK
The Ultimate Additional Japanese Resources List!
The New And Improved List Of API and Third Party Apps
I hope your Japanese learning journey goes well and that you enjoy your time with us on the forums.