One of the major misconceptions may people are under is that SharePoint is simply another files and folders repository. SharePoint is a collaboration solution and therefore a very different beast from what many may be familiar working with.
A good example is the issue around the modified field. Let me explain.
One of the many things that SharePoint allows is the mapping of a document library using Windows Explorer as you can see above. You can easily drag and drop files into a SharePoint library using this interface.
In this case let’s pay special attention to the Date modified for each file shown above
If you now look at the file listing in the web interface of SharePoint after copying the files across using Windows Explorer you will see that the Modified field is different in the web interface from that in the Windows Explorer view. Why is that?
The reason is that SharePoint is a collaboration system not simply files and folders storage as I said before. What does that mean? What it means is that when a document is copied to SharePoint it is now only part of an item in a list of items that together constitute a Document Library.
What that means is that in SharePoint an item is not only just the file but also things like previous versions, workflows, additional metadata, check in status and so on.
If we take a look at the properties of another item in another SharePoint Document Library that includes some of these additional elements you’ll see what I mean.
So for example, the above item includes a file called 0 – general index.docx and Title but it also contains an additional field called Customer. Thus, this Document Library item is composed of a file AND metadata. So if any of these elements of the item change then the Modified field for the whole item will change.
Thus, in a SharePoint Document library the file inside an item can remain unchanged but other information around that file in the item can change. These changes can trigger things like workflows so it important that a change anywhere in the item is reflected by SharePoint.
So in summary, remember SharePoint is not simply a files and folders replacements, it is a collaboration system and therefore different. Uploading a single file into SharePoint adds that file to an item in SharePoint that contain additional information such as metadata. Thus, any uploaded file is only part of the item in SharePoint. The Modified information you see in the web interface for SharePoint refers to the whole item, whereas the information you see via Windows Explorer (which is a much simpler client than a browser) refers directly and only to the file because it can’t read any of the additional item information from SharePoint.
I hope all that makes sense.
CIAOPS 
