"It was registered three months following the event. Why might they have waited so long to report it?"
Why would a Jew register their births at all in Russia (or elsewhere in Europe)? Birth records were used to draft boys and tax families. Civil marriages were sometimes heavily taxed or even banned, so the children were recorded as illegitimate. So why would you do it.
In the US today, almost all births are in hospitals, so it is done for you, by the hospital (or midwife, if she is licensed); this is the law, and nobody objects to it. Russia was different because nobody trusted the government.
I have seen records of people with birth records registered 20 or more years later, because, perhaps, they need it to marry - if they want to marry civilly. I had a (female) cousin who was a lawyer in New York nearly 100 years ago, and she had to get a birth record from Russia to prove whatever to take the bar exam. I have a copy of it. It wasn't done at the time of her birth but long after, but it satisfied the requirement, apparently.
Sally Bruckheimer, Princeton, NJ <sallybruc@...>