I would suggest pointing to the nature of the Intel® Turbo Boost technology. This feature is auto-managed by the processor having a balance between the power consumption and the chip heat.
Also, the maximum Turbo Boost speed won’t be reached by all of the processor cores at the same time, the value is assigned to the cores being used gradually at the moment the Turbo Boost kicks out so it is possible that only one core reaches the maximum Turbo Boost speed assigned by default.
If you change the Turbo Boost frequency multiplier manually then the processor will try to apply it to the cores in use but if the maximum power allowed (TDP) is already covered then the CPU won’t continue increasing the cores speed to prevent heat problems. Since you are setting a new value manually for the Turbo Boost frequency multiplier it is possible that the CPU is finding a power limit when reaching the 3.5GHz.
If you compare this Xeon processor (max TDP 69W) with the I7-3820 (max TDP 130W), the behavior will be different because the architecture is not the same. The Intel I7 processor is able to get to the maximum turbo frequency since it has higher TDP.
Again, you can change the settings, but we do not recommend.