Apple - 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.

On desktop, Hyper-threading is a feature of i7. In the low-power laptop segment, they i7/i5 naming is mostly marketing. So the i7-7200 is just a bit lower clocked version of th i7-7600. Here a overview on Wikipedia