Honestly, the first site is fake. As for the others I am unsure they are right as. I could put my friends birthday and mine who share the same day of birth as me and say the same thing. I do not think the hundreds of people I share the birth with shared the same past lives. Meditation and studies is how to find out your debt and to have no debt means that this life and your previous life that you have not done anything wrong since look into the Vedas and other philosphies that speak of reincoarnation.
One thing to always bear in mind is that in numerology, the numbers are symbolic to particular energies and concepts. So something like karmic debt is not tracked in the sense of some cosmic coach keeping score and saying "You have three things you need to fix."
And to help clarify what was said earlier about the site likely being fake (or at least inaccurate) is that in numerology it is difficult to produce a zero. Any translation I have seen (Hebrew kabbalah is a favorite thanks to Linda Goodman) always starts with 1 for translating letters, and no birthday can ever contain less than 1 as well. The only way to achieve a zero value is through reduction, and numerology doesn't do that, as far as I am aware.
It may be in how the site you got that from reduces numbers with multiple digits. The site might be short-cutting by just taking the last digit of whatever input they receive or calculate that ends up more than 9.
In numerology any figure is a combination of the energies of the individual numbers within it. For example, the year of my birth is 1980. so that means the influence of that would be a combination of 1, 9, 8, and 0 energy. Because they are combined, they can be compressed by adding the individual digits together. 1+9+8+0 forms a new digit of 18. Again, that number would be compressed (1+8) to form the single digit 9 and that would be the one I use for my desired interpretation.
In some schools, the double-digit figure can also have significance if it is a prime number like 11, 13, 17, 19, etc. So you would want to note that number if it appears before compressing to a single digit.
Zero energy itself tends to be interpreted as representing the classic concept of 'everything and nothing'. Think of it as the white light of numbers. It is a combination of all of them while representing none. Adding more zeroes to any number doesn't change the energy of the number. Which also means that zero on its own represents energy itself, without influence or form. it can also be interpreted as representing cyclical nature, and how beginnings and endings are one and the same.
It has been a while since I have flexed my numerology muscles, but if you want I can dig out my books and put together some sort of a chart or breakdown. Or if you would prefer I can also throw up a list of books you might be able to study from to be able to put a chart together yourself in the future. In all likelyhood if I make a chart for you I'd also include the reference material I used regardless. ^_^
I don't know much about 'karmic debt numbers' as i am not into numerology.
So reincarnation is an interesting concept but not everyone believes in it. I know people who believe one can have past lives as other animals. Some reject the concept claiming we only live once and then move on to another existence upon the event of 'death'. For me the latter makes sense only if we originate from that same existence, say before birth?