You can read more about our word selection algorithm in the docs here.
To highlight one important factor:
- Do all characters have a word item to pair with it? Do some characters have less word items than others?
In short, we always select words to help with learning new characters first. So, as long as you continue to learn new characters, words will be prioritized to make sure you have “word coverage” for those new characters. I understand that the default ordering of words on that list page can be misleading though, we should change that 
Here’s another thread on the topic: