The difference might be a single bit or byte but it's difficult to tell from the sizes shown in search results. The difference might be related to a difference of description such as in id3 tags though often these are even grouped. Also, for the example you gave, the encoding might be slightly different having used a different encoder. eg: .mov can represent quite a few different variations of format. A QuickTime movie can be encoded using many different types of codecs but the final file might use .mov