Different vendors have different ways of sending tracking information via email.
For example the following link was sent to me from Apple:
http://store.apple.com/trackshipment/W*****1/*******6371
... which obviously loads an Apple web page which in turn will provide Apple's own tracking information. Other vendors might send a link to a FedEx web page, or a USPS web page, or some other third party website that provides tracking information on the vendor's behalf.
The point is that there are many opportunities for errors to be introduced. To avoid those errors you're better off just copying the actual tracking information and pasting it into the carrier's own website.
It's an enormous oversimplification and unnecessarily cautious advice, but this is one reason I typically recommend never click links sent in an email message. Copy and paste if you wish, so that you can independently verify the link's accuracy.