Firefox is worse; it often immediately goes to about 75%, then just sits there, then jumps to the end; it is the most useless progress bar I have seen yet (and I have been using computers since Windows 3.1).
A small file could cause this. The browser starts to download the file to a temporary directory, while the user is choosing the final location, so when the progress bar appears, most of the file is already there, then the rest of the file arrives at the same time that the system is creating the new file and copying the contents from the temporary file to the location chosen by the user. Disk file operations can block for an instant. When the bar is to be updated again, the download is complete, so it simply vanishes.