Are you taking into consideration when they use abilities/weaponskills? Those actions may very well cause a delay in auto-attack. Also, take into consideration that the actual attacks may not be perfectly synchronized with the video's "timer". For example, an attack may occur on the 55.6 second mark, but the video isn't going to tell you that, it's going to say 55 sec. This could throw off calculations.

In short, it's probably not such a good idea to deduce detailed information from a crappy YouTube video, who knows what factors are coming into play in this situation.