Why do I see strange new keys marked as ONE, FEW, MANY, OTHERS?

Are you seeing something like this?

Some i18n formats, like i18next are able to handle languages having multiple plural forms too.
Locize automatically shows all needed plural form keys. For example in the screenshot above the keys: someKey and someKey_plural were added and locize automatically shows also someKey_0, someKey_1 and someKey_2.
You can use this small i18next utility to check the correct plural suffixes.
The i18n format depends on your i18n framework.
If you're not using i18next, you might change the i18n format in your project settings: