More Vdd than Vss Pins
As a chip maker, it's easy for me to explain the cause of the imbalance. It's that there are several different rings of VDD in the IC for different purposes, but only a single ground. The different VDD rings can be in different voltages, but the ground is always at zero volts. 

So for the ground, there is a solid copper rectangle in the leadframe (that's what the IC pins extend from) under the die for the ground. Internally, there can be dozens of pads that are all down-bonded to the ground copper. This way the ground can be quite solid across different parts of the IC, which minimizes substrate currents - current flowing through the copper will not cause problems such as latch-up in the IC, unlike strong substrate currents that cause latch-up conditions. 

So, internal to the plastic casing in the IC, there are, more or less, the GND/VCC pairs that you mention in your question. But as for the ground, due to the ground pad in the leadframe, not every GND pin needs to extend out from the IC package - the ground copper inside the IC package is strong enough.