Many of these "numbers" have been verified by experiments and observations, and they are continually being measured to higher precision. This latest one comes about from the prediction of the amount of certain gases during the early evolution of our universe.
But more data has just come in! Two new measurements, in a paper just coming out now by Signe Riemer-Sørensen and Espen Sem Jenssen, of different gas clouds lines up with a different quasar have given us our best determination of deuterium's abundance right after the Big Bang: 0.00255%. This is to be compared with the theoretical prediction from the Big Bang: 0.00246%, with an uncertainty of ±0.00006%. To within the errors, the agreement is spectacular. In fact, if you sum up all the data from deuterium measurements taken in this fashion, the agreement is indisputable.
The more they test it, the more convincing it becomes.