It's 3x the difference:
I-864 instructions, part 7, #10, first paragraph:
10. Total Value of Assets.
In order to qualify based on the value of your assets, the total value of your assets must equal at least five times the difference between your total household income and the current poverty guidelines for your household size. However, if you are a U.S. citizen and your are sponsoring your spouse or minor child, the total value of your assets must only be equal to at least three times the difference. If the intending immigrant is an alien orphan who will be adopted in the United States after the alien orphan acquired permanent residence, and who will, as a result, acquire citizenship under section 320 of the Act, the total value of your assets need only equal the difference.
I would call the NVC and ask to get more clarification. Did you submit a explanation as to why you didn't file your taxes? Did you provide enough proof of your assets? Maybe they don't want to approve because it doesn't appear you have continuing income as you haven't worked the past three years and haven't filed taxes?
The three numbers after the 2015 is the julian date it was assigned plus 500. So if yours is 621, then 621-500=121, and the julian date 121 is April 30. So your case was assigned April 30. Your friends was assigned April 22.
The last three digits are the number that was assigned that day. So if yours is 2015621150 (for example), then it was assigned on April 30 and it was the 150th case to be assigned that day.
https://docs.oracle.com/cd/E26228_01/doc.93/e21961/img/image121.gif
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