Even though they are both used, there is an official one and those that are not.
When you consider the history of kanji incorporation into Japanese, first, there were Chinese writings. Then, people tried to read them as Japanese. Two techniques appeared: (i) kaeri-ten, which marks how the Chinese characters in the original Chinese writing are to be permuted when read so that the Chinese writing can be read as if it is Japanese, and (ii) okurigana, which marks the inflectional endings that vary in Japanese but not in Chinese.
Hence, the purpose of okurigana is to mark the varied part of a word. This works for verbs. Now, once a verb is turned into a noun, and starts to be recognized as a noun that is independent of the original verb, there is no more purpose to keep the okurigana. That is where the reduction of okurigana happens.
Variation among the okurigana reflects the fact that it is not clear when a deverbal noun is created: some people might feel a word is still an inflected form of a verb whereas other people may feel that the word has evolved into an independent noun and has lost connection to the original verb. There is an official one stated, but people have varied perception.