The amount of B12 your body needs to consume from the diet is very individual, but the vitamin labelling rules don't allow for this.
The entero-hepatic cycle is a special recycling process where B12 in the blood is collected by the liver, passes down the bile duct and into the intestine, and then gets reabsorbed into the blood. We don't know why, but it's likely that the liver takes inactive B12 from the blood (in storage) and the intestine activates it when it's reabsorbed. ie to convert storage B12 into active B12.
In most people, this entero-hepatic cycle is very nearly 100% efficient. Therefore most people only need a tiny amount of dietary B12, because they are reabsorbing so much from their entero-hepatic cyle - say 6mcg/day.
We estimate that there's a total of about 30mcg in the body - around 91% in the cells, and only 9% (2.7mcg) in the blood. However the entero-hepatic cycle is a busy one and churns the B12 perhaps 24x per day (average once per hour) - in other words, 65mcg of B12 goes through the cycle per day. At 90% efficiency, we'd expect 6mcg loss in other words you need to make it up with 6mcg from the diet, fairly easy.
HOWEVER people with B12 deficiency are mostly far worse at absorbing B12. At worst, with no active absorption, passive absorption is 1% - in other words a 99% loss. That means that of the 65mcg circulating into the gut to be activated and reabsorbed, 64.4mcg is lost and needs to be made up from the diet. Let's take an average value for a person with B12 deficiency, 5% efficiency (still 5 times the worst case). Every day, they lose 61mcg. But when they take 1000mcg in a tablet, they only absorb 5%, in other words tehy only absorb 50mcg - so they aren't getting enough from the tablet to make up the amount lost.
Daily Values are based on the assessed needs of a healthy person who doesn't have absorption problems. Because of this, the Daily Value for B12 is set at around 6mcg per day. But
Injections put the B12 directly into the blood so they aren't affected by the absorption efficiency (well, they are to activate). That's why we say that you don't need to inject very much, you just need to inject it more frequently.
You have around 10% of your body weight as blood, in other words if you weigh 80kg then you will have around 8lt of blood. Therefore if the ideal blood serum B12 is 800ng/L (and blood cells have about the same concentration of B12 as the serum), then you will have 8lt * 800ng/L = 6,400ng = 6.4mcg of B12 floating around in your blood. This means that if you inject 1000mcg (the usual amount), then you have temporarily increased the total B12 in the blood from 6.4mcg to 1000mcg. Within seconds, a lot of this will be absorbed into cells to boost their B12 levels, but also a lot will go to waste out of the kidneys.
If you lose a lot through the kidneys, then there's no point in putting more in with each injection because you'll lose most of it. It's far more sense to give yourself another injection a few weeks later. Personally I've tried with injections of 500mcg (0.1ml of 5mg/ml methylcobalamin) and 250mcg (half again) and settled on 500mcg per injection simply because it's quite difficult to load the syringe with less and still manage to inject. But I inject twice per week.