Could be unfamiliarity with "twenty to" or even with not using the 24-hr clock, but actually lack of context could also be a factor. It would probably be obvious, if you had the video, if it's not 10pm, but not if the translator's not given that visual context. Again, the initial mistake could be nothing to do with the translator but a mishearing (or misunderstanding) by someone creating English subtitles. It all depends on the process used, but someone involved being unfamiliar with that usage does seem a likelihood.