The fact is that the traffic limit is taken into account not only for downloads, but also for watching videos online. Also, if you started downloading a file, but canceled it in the process, then the entire file size will be taken into account in the limit. The same with online video viewing: if you watched 1 minute from an hour-long video, then this file will be considered as a whole downloaded.
Please note that when receiving a link, some managers follow the link in the background to estimate the file size. Because of this feature, the full file size will be taken into account in your daily limit.
If you watched the video online, then when downloading the file will not be taken into account in the limit and vice versa. In other words, the file does not count in the limit twice during the day. The traffic limit is reset at 0 GMT.