Q, I'll try to explain this as much I can in simple words .
Section 201 of the Immigration and Nationality Act (INA) sets an annual minimum family-sponsored preference limit of 226,000. The worldwide level for annual employment-based preference immigrants is at least 140,000. Section 202 prescribes that the per-country limit for preference immigrants is set at 7% of the total annual family-sponsored and employment-based preference limits, i.e., 25,620
This total includes FB + EB , for a non-retrogressed country this annual limit is never reached . DoS has no botheration over monitoring the visa numbers released for the "C" countries.(those countries whose dates are always current in the Visa bulletin)
In the case of both India/China, they have backlog in each and every sub-division of FB and EB. DoS has no option other than fixing 7% upper limit within the subdivision of FB and EB. This way they can monitor that 7% of F1 + 7% of F2..........+7% of EB5 will not exceed the 25,620 .
For India, 7% of Eb2 limitation is 2800 ( in a typical 140K year).
Out of 10,800 released per quarter to all EB2 countries, only 27% of 2800=756 is allotted to India per quarter. This 756 is subjected to annual limitation.
If all 10,800 are not seem to be utilized, CO by law has every right to distribute to backlogged countries. These extra visas are not counted against that 2800 total.
Again, India is eligible for 756 in the second quarter.