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I assume you are referring to the 99.7 MHz reading in CPUz, which "should" be 100MHz?
It is normal for the "CPU FSB" (actually called BCLK, or Base Clock for your CPU) to vary somewhat from the nominal value of 100MHz. That's because the oscillator creating that frequency is not perfect, and small changes in power to it may cause it to vary. I see this with my i5-3570K, and other CPUs and mother boards. It is not a problem or sign of a defect, and does not harm anything.
You could try to set the value of BCLK in your boards UEFI/BIOS to 100.0, instead of Auto, or increase it a little to say 100.2, so it stays at 100.0 most of the time. It will likely still vary up and down a bit, even if you do that.
I suspect that you are really concerned about your CPU running at 1595.9 MHz (~1.6GHz), instead of its rated speed of 3.4GHz. That is not caused by the slightly lower BCLK.
Notice in your picture where is shows CPU Multiplier: 16. The clock speed the CPU is operating at is determined by the CPU multiplier and the BCLK, which is the multiplier x BCLK, or 16 x 100MHz = 1600 MHz (actually 1595.9 MHz, due to the slightly lower BCLK.) The CPU Multiplier affects its speed much more that tne BCLK being a little low, or high.
Why is your CPU multiplier at 16, when it should be at 34, for a speed of 34 x 100 MHz = 3400 MHz? In your boards UEFI/BIOS, the CPU power saving option called SpeedStep must be enabled. When the CPU is under low demand or load, it will automatically change the multiplier to 16 to save power and allow the CPU to run cooler. The multiplier will (should) change to 34 when the load on the CPU increases. Note that each core of your CPU can adjust its multiplier independently, so one or more may be at 16, while others are at 34.
If Turbo is enabled, the multiplier will go up to 38 when the CPU is under a higher load.
CPUz is only showing one core multiplier, and apparently is not updating its display, since the multipliers change very quickly up and down, all the time. There are other hardware monitoring programs, like HWiNFO, that will display each core independently, and you can see them change all the time.
It is possible that the CPU multiplier has been set to 16 in your UEFI/BIOS, rather than Auto, which would cause the multiplier to stay at 16. It's much more likely that SpeedStep is enabled, which is commonly set as a default.