I can answer that.
Imagine something like this:
Each day you have about 6 customers, on average. The first customer is gonna buy 3 grams but you wont see him for a whole week. A second customers pulls up and he buys 1 gram but later in the evening he will buy 1 more gram and he does this everyday. The third customer is a reseller, he buys 10s of grams each time, usually buys 50 grams but you dont see him for a whole month, can be months even between purchases. And the rest 3 buyers are loyal customers, one is a college student, one is a club bouncer and the other is your grandma because shes a badass, they all buy 1 gram, once a week.
Now add all of that up:
Daily you sell something like this: 2 grams (2nd customer, each day he buys)
Weekly: 3 grams (1st customer) + 3 grams (those three people, grandma, bouncer and the student) + what you sell daily (2*7 = 14) == 20g
Monthly: 50 grams (the reseller) + weekly sold ( 20*4 = 80) == 130g per month!
A year you sell: monthly * 12 == 1200 grams
So this is kinda the math behind it, the more customers you have the better, focus on selling high quantities if you have a kilo, you want to become a supplier not a street level dealer.