MacBook 12'' 2017 cpu speed: 1.4 GHz but Intel says it's 1.3 GHz
The key phrase that you alluded to was 


  "...shows the base frequency as 1.3GHz"


(emphasis mine).

From the Intel Ark spec sheet link you provided


  Processor Base Frequency
  
  Processor Base Frequency describes the rate at which the processor's
  transistors open and close. The processor base frequency is the
  operating point where TDP is defined. Frequency is measured in
  gigahertz (GHz), or billion cycles per second.


(TDP is Thermal Design Power - or the wattage it dissipates with all cores running at normal operating frequency - the "baseline" if you will)

A little bit further in the Specs you will find the following values:


Configurable TDP-up Frequency           1.60 GHz
Configurable TDP-up                     7W
Configurable TDP-down Frequency         600 MHz
Configurable TDP-down                   3.5 W


What this is telling us is that the CPU can be configured up or down by tweaking the speed and TDP values.  It can go up to 1.6 GHz (1.4GHz is within this limit) or down to 600 MHz.  More speed, more heat, more power consumption; less speed, less heat, less power consumption.  It's up to the integrator to decide what makes most sense for them.

Apple made the decision to tweak it to 1.4 GHz.