Accessing Excel Files on SharePoint from Tableau

Recently, for the first time I had to connect to an Excel file on a SharePoint site. I was told that by opening Tableau's new data source interface and selecting Excel, I just need to add the URL to the file, such as http://server/site/library/filename.xlsx

It did work at first! We get all the data etc. Then strange thing happened. The data source won't update by refreshing data source, even when we made changes in the Excel table. By searching around and I found the following articles which are very helpful:

UNC Path Naming for files stored on SharePoint
Connecting to SharePoint-Based Excel File

Here is the solution:
Instead of using the URL link, we must use UNC link, which is very similar to the URL: just reverse the slash to be back slash (minus http:)
\\server\site\library\filename.xlsx

What is going on?
The culprit is that the http link tends to store the Excel file in a local cache. Then the file content won't get updated as quickly as we wish. UNC link is linked to the original file without any caching.