Vitamin pills are different from stuff for injection -
1. Vitamin pills are not subject to the stringent testing and regulations that medicine for injection will.
2. Vitamin pills can, and will, contain widely varying amounts of the 'active' ingredients and excipients.
In the UK, unless you are selling B12 that is not for injection, or not for medical purposes, then the stuff you are selling will be virtually identical from different manufacturers.
How do I explain your anecdotes - it's the same as the way that blue pills help people sleep better than red pills.
People will take Brand X of a product for quite a while. Each time they take it they will, through complete randomness, feel somewhat better or worse than normal.
One day they switch to Brand Y - an identical product but with a different label on it. It just so happens that they are feeling particularly good on that day. They notice it, because they've done something new, and ignore the golden rule - correlation does not mean causation. They swapped from Brand X to Brand Y and felt better. Therefore Brand Y must be better that Brand X.
So they keep taking Brand Y. And they keep feeling better on some days and worse on others. But they know that Brand Y is better than Brand X - so they ignore the worse days and remember the better days. It's called 'confirmation bias'.
It's why real scientists tend to ignore anecdotes and poorly-designed studies and concentrate on peer-reviewed double-blind studies on large numbers of people.
In the absence of such studies then individuals can, if they are honest with themselves, form an opinion. They can do so by keeping a record of symptoms and treatments. And objectively examining the results to look for any trends. (Objectivity is best attained by imagining that somebody you dislike is putting forward a proposal - Brand Y is better than Brand X - and you're trying to disprove it). That's how I realised that methylcobalamin patches were not making any difference for me - and that methyltetrahydrofolate pills were.
But customer anecdotes are notoriously inaccurate. Just read through the Amazon reviews of some products that you own.