The key phrase that you alluded to was
"...shows the base frequency as 1.3GHz"
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.