One of my 6 month olds is only 55 grams, but that's because she was a VERY small 27 grams hatchling that refused to eat on her own until month 4.
Size can vary by so many things, as mentioned above, some breeders do feed smaller meals on a longer period as a maintenance meal. Weekly feedings of 10% are meant to keep the animal growing at a stead pace. One of the snakes I received was a year old and had only weighed 200 grams. One of my current 6 month olds, a sibling to the runty 55gr is 170 right now.
Or the other issue could be this was a small or problematic hatchling who did not get a strong start. Maybe it took a while to get feeding regularly. As long as the snake is steadily eating and has had at least 5 meals, that's usually considered a good selling time
A good breeder should be able to answer any concerns you have on the size.
There really isn't a perfect size guide for the different ages. It just depends on the individual animal.
Another thing is as mentioned, try to keep it on the same food the breeder is feeding... But keep in mind that sometimes even though you are feeding the same, the animal may still be stressed and refuse to eat. You won't want to handle them until they have eaten at least 3 good meals for you to help keep them on pointe with feeding