All

All of those numbers are precessional. Meaning they are related to the precession of the equinoxes.

The 4-3-2-1 ratio of the Yuga periods, the 4.32 billion years of KALPA, the 4.32 million years of a MAHA-YUGA, and so forth....these are relating to the number 4,320. Divide that by 2 and you get 2,160, or the length, in years, of one of the ages in the precession of the equinoxes.

The MANVANTARA is divided into 71 MAHA-YUGAS. 71 is also a precessional number. It takes 71 years for the precession to shift one degree of arc in the sky.

To demonstrate, right now the most up to date number I can find for the full length of a precessional cycle is given as 25,771.57534 years. This value is reached using data from the latest instruments and using corrective procedures based on modern astrophysical concepts and other related sciences.

25,771.57534 divided by 360(to give us the number of years required to move one degree of arc) equals 71.5877092778. Rounding down to the nearest whole number is, of course, 71.
(72, an equally valid rounded number from 71.5877, is also related to precession and is found in many disparate myths, legends, and ancient texts. The 72 diciples sent forth before Jesus. The 72 forms of transformation learned by the Monkey from the Serpent-god in Chinese mythology. The 72 virgins said to be in the afterlife harem of a martyr for Islam. The 72 demons of Solomon. Just to name a few.)

Going back to the modern value given for the length of a complete cycle, if we round that value to 26,000 years, that gives an age length for each of the twelve ages as 2,166.667, which is very close to 2,160, or half of 4,320.
Yes, there is a lot of "convenient" rounding of numbers involved there, but remember that these numbers were inserted in mythological tales meant to be conveyed often only by word-of-mouth, across thousands of years, and 2,160 is a lot easier to remember than 2,147.6312783(the actual length of an age, without any rounding), and 4,320 is even better.

To illustrate, in the Nordic myth of Ragnarok, when the forces of good are marshaling to go forth and fight the final battle, there is this passage:
500 doors and 40 there are
I ween, in Valhalla's walls;
800 fighters through each door fare,
When to war with the Wolf they go

So there are 540 doors, and 800 warriors behind each door. So the forces of Valhalla number 432,000 warriors in all.

Precession.

I point this all out because it fascinates me. I have no idea what it means, and am not implying any different meaning to the Yugas thereby. Its just that these same numbers, undobtedly related to the precession of the equinoxes, are found in all the most ancient myths and tales, from all over the world. I had no idea they were present in the Yuga stuff too.

Amazing.