Baby weight is measured WRT centiles, not quartiles. The entire concept of centiles is never explained and many people are confused about them because unless you've studied statistics it's not the kind of thing which exists in general knowledge.
I know when DS was born I was forever trying to work out which "centile" he was on when he was between two lines, assuming that they ran from 0 to 100 with every number in between, it doesn't work like that, at least in health related weighing - I have no idea how they work in other contexts. (Crash course follows.) In baby weighing you have discrete centile lines, which is the important part, the currently used UK ones are 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th. Each one of these nine centiles is counted as one even though the gaps between them seem to vary.
The current charts being used are based on predominantly breastfed babies. They also exclude parental smoking. They're intended to be a measure of ideal child growth rather than being simply an observational tool.
"The WHO standards establish growth of the breastfed infant as the norm for growth. [...] The WHO charts reflect growth patterns among children who were predominantly breastfed for at least 4 months and still breastfeeding at 12 months."
This comes from the CDC but refers to the WHO charts developed in 2006, currently used by the NHS and which most PCTs switched to by the end of the 00s. I know this because DS was born in 2008 and he had the old chart but there was lots of discussion online about them changing. I expect each PCT used up their stocks of old books at a different rate but it's now 11 years later, everyone should be using the new charts. If you do want to double check they are available online for anybody to access: UK charts You can also see the training materials in how to read them here: www.who.int/childgrowth/training/en/
If you look at the new charts you will notice that there is a gap between birth and two weeks of age. The old charts don't have this. This is to reflect what we now know about birth weight which is that it is largely irrelevant to a child's natural size and is more related to the size of the mother, the gestation of the baby and to some extent the weight gain of the mother although this is mainly related to gestational diabetes.
Basically, babies can be born smaller or larger than they are "meant" to be and then spend some time catching up or catching down. By two weeks they should be settled on their actual centile, and from here, it's normal that they will move above and below it a small amount as people don't tend to grow in a linear fashion. It's important to remember that growth charts show an average of babies, not "the" average baby. Nobody is average. Don't expect your baby to follow the line, no baby follows the line, especially if you get them weighed very often. (With less frequent weighings, you'll see weight patterns smooth out). They will dip over and under it, and it's normal and expected. A problem is classed as a baby's weight gain line crossing two centiles, and even then, it can sometimes be okay for this to happen, perhaps if a baby is ill, they will temporarily dip as their appetite reduces and then come back up, or sometimes settle on a lower line than previously, but still be healthy.
IME most health visitors don't explain any of this, some will inform you that your baby has dropped or gained centiles, as though you might already know what this means, when most people don't. Some HVs even seem to have a poor understanding of centile charts themselves which I find a bit baffling, as they are not difficult to understand if you have the information, which when it's your job, you really should.