You can find those multiplier in cpu-datasheet. For 2-core load you calculate frequency as:
base_frequency_[GHz] + (multiplier * bus_clock_[GHz]) = 2.3 + (13 * 0.1) = 3.6 GHz
For 1 or 2 core load, multiplier is 13 (that number sequence starts with load for all 18 cores, then 17, 16, 15, etc, down to single core). Now in your case, VM has 4 vCPU. You'd expect to use multiplier 10 and get 3.3GHz, but nope! Multiplier 5 is used. Why? Because of the feature called "thermal load ballancing": ESXi "rotates" that 4vCPU load over all pCPUs/pCores, so that all pCPUs/pCores are moderately loaded (therefore in your case multiplier 5 is used). Every modern OS does this (even Windows). Otherwise some portion of CPU-chip would be extremly hot, while other parts were cold.
If you had Windows installed directly on physical machine, you could prevent it using "cpu affinity". You can probably do it in ESXi too, but I DO NOT recommend it, unless you are ready to burn/wreck your pCPU. If only 2 of 18 pCores (on each pCPU) are at 100% and remaining close to 0%, thermal stress on the boundary of loaded and non-loaded cores is extremely high. This could really over-stress pCPU up to the point leading to physical damage (even with very good cooling), if you run it for longer time...