During watching some movies, it happens that we have to download the subtitle files(srts) from different sites.
In a single site, we can find dozens of English srts for a single movie. But the srts are different in some way and the main difference which I found is the timing of inappropriate srts don't match with the timing of the movie i.e. either the srt leads or lags when the movie is being played.
So what are the factors should we take into account during downloading a proper subtitle of a proper file of a movie? Why is there a mismatch between subtitle timings across different subtitle files?