You bolded the varying part so i assume you're confused about that. Power saving features. The multiplier should be varying also. It should hit 1666MHz on load.
You are using the intel socket M. First, I don't recommend upgrading a laptop due to the work required (disassembly etc. is much more difficult than a desktop) and the fact that the CPUs are harder to find and more expensive if you can find them. I am in Canada and here, no retailer I know of sells mobile chips.
If you want to upgrade still though, you will need to go on the manufacturer's site and look up your series of laptop. There they will list the CPUs they put in them. You can look and see and know that all the CPUs listed will work for your setup with power and heat not being an issue. You will also know that the chipset and bios will support them.
You can upgrade to basically any other "t series" chip with the same thermal/power (TDP) requirements. But as I have always said in the laptop forum, it can be rather difficult to change out a laptop CPU if you do not know what you are doing. More power to you if you do, but it can be a PITA.