With Google accounts the tracking pixel may not be downloaded or may show strange open results. Google opens and proxies images transmitted to its users and uses a cache mechanism to serve a link to the same image.
Because Google proxies the image shortly after receiving the email, the initial open notification will often be a result of this action.
Google decided to use this method of image expression to save users and their server's bandwidth, avoid duplicates and optimize the client-to-server-to-client image processing.
Basically, it stores a copy of an image, and then, if some other Gmail account requests the same image, it is taken from the copy instead of the original location. Since most tracking software can only detect a download from the original location, the open tracking doesn't work or shows strange results when Google first retrieves the image (an "open" that is not made by an end user) and serves the cached version of the image from their image proxy server (end-user opens that may not be registered).
More details on this can be found on Google's official blog and this post about Google's image proxying from an external URL.
Most tracking software is highly inaccurate due to these systems as well as user error i.e opening the email in their own sent folder.