Windows – PC with Windows 8.1 boots from any key pressed

This is a known issue: http://support.microsoft.com/kb/2719667/en-us?sd=rss&spid=14498 (funny enough I just happened to see it show up recently in the knowledge base RSS feed)

If an application is preventing Windows from shutting down (ex. unsaved work), Windows will wait a period of 60 seconds at the force shut down dialog before forcing the application to close so shut down can continue. If the user cancels out of the force shutdown dialog, Windows still retains the 60 second timeout value. As a result, the next time a shut down occurs, even if there are no blocking applications, shut down will be delayed until the timeout value expires.

This behavior only occurs when configuring the power button to shut down the machine. When shutting down Windows through the Start Menu, the problem does not occur as the timeout value is not retained when cancelling out of the forced shutdown dialog.

Unfortunately the work-around is:

To work around this issue, use the shut down option from the Windows Start Menu instead.

It's the same.

Like you said, you can force a shutdown by holding down the power button for longer (approximately 4 seconds), but this is literally cutting the power. An ACPI-initiated shutdown is the same as one you call through software. Both ways, all applications running on the machine have to be informed of the pending shutdown, (possibly abort it, depending on the version of Windows), and close themselves and any files they have open gracefully. That all takes time.

Basically, what makes the magic happen in either case is a call to the ExitWindowsEx function, which is part of the Windows API used by programmers.^* If you specify the EWX_POWEROFF flag, the system is shut down and the power is turned off. It is also possible to call that function with the EWX_FORCE flag, which will do just what it sounds like—force a shutdown without giving running applications the ability to delay or cancel it. If you do that, shutdown will likely happen faster but it will not be a completely "clean" shutdown and is therefore dangerous.

The difference in behavior that you're seeing is probably due to way that the software shutdown is initiated. For example, when you execute shutdown.exe, you can indicate a timeout value for when the shutdown request should be initiated. During this period, the pending shutdown can be cancelled by running shutdown.exe again with the /a flag (to abort a pending shutdown). 60 seconds is a pretty common value for this timeout period.

The rest of the difference is probably just perceptual.

_{^* The fact that Windows is a multi-user operating system can make things somewhat more complicated. But that's getting beyond the point here. Any other method of initiating a shutdown (e.g., InitiateSystemShutdown) is going to behave similarly.}