As of 1875, the onus was on the parents (or whoever else was present at the birth) to register a birth, as from July 1837 to 1874 it was the job of the registrars and their deputies etc, to get info on births.
6 weeks was given to register a birth, 42 days. With the birth certs I have, the time frame within those 6 weeks varies depending how on the ball the registrars were at getting the info, and from 1875, depending how quickly the parents registered the birth. I have some where the birth was registered just within the 42 day timeframe, and others registered within days. Although some were registered late. I have one born 1st June 1899 and registered in August, so the parents must have paid the fine for late registration.
For people new to genealogy and not sure how the system worked, they may get confused if say for instance they wanted to look up their own birth in the indexes (due to lost birth cert etc) and was born in for example, 15th December 1964 in Lambeth but cannot find their birth in the Oct/Nov/Dec quarter of 1964. If they was at the FRC or at the Kew archives in person looking at those ledgers, they would have to ask at the desk if unaware of the 6 week period allowed to register a birth, but if on a laptop and they typed their name and mothers maiden name into Anc, FreeBMD or FindMyPast and expected to find their birth in the Oct/Nov/Dec quarter of 1964, but found it listed in the Jan/Feb/Mar quarter of 1965, they would wonder why their birth appears late. They then look at the original index entry and it says "Births registered in Jan, Feb and March 1965". They bought a copy and the birth is correct, mid Dec 1964 but registered 5th January 1965. They would probably then understand the GRO indexes go by date of registration, not the date of the event.