RealTemp is the only program of the bunch that follows the monitoring method recommended by Intel. It uses high performance timers within your CPU. These timers run at billions of cycles per second so software can precisely determine what multiplier a CPU is using.
CPU-Z might start with this method but then it modifies the Intel recommended method so it can provide users with consistent MHz numbers for validation purposes even when a CPU is lightly loaded. If a CPU is using any of the low power C States like C1E, C3 or C6, the multiplier can be rapidly changing thousands of times a second and CPU-Z has chosen to ignore this. Reporting either the highest Turbo multiplier or the 16 multiplier when lightly loaded and nothing in between does not tell you what your CPU is really doing internally.
If RealTemp shows your multiplier jumping all over the place it is because your multiplier IS jumping all over the place.
Every monitoring utility gets compared to CPU-Z so some of them have decided to try and report the same as CPU-Z does. It's a lot less hassle doing that than being criticized in forums for not being just like CPU-Z.
Intel includes high performance monitoring timers in their CPUs for a reason and these timers are available in all of their Core i and Core 2 CPUs. Intel also publicly documents how to go about using them. That's the method that RealTemp has been following since November 2008 when Intel released their Turbo White Paper.
If you want to see what some of the popular monitoring utilities don't tell you about your CPU then check out this post.
You may not realize this but CPU-Z changed its monitoring method recently.
What version is correct? Does either version accurately tell you what your CPU multiplier is really doing?
The monitoring timers in the Intel CPUs are a shared resource. That means if one monitoring application decides to stop and start these timers randomly, that can interfere with other monitoring software getting accurate results from them. I can guarantee you that if you use RealTemp by itself, it will be able to tell you exactly what your CPU is really doing.
RealTemp T|I Edition