It is VERY easy to be mixing a formula (a "recipe" if you will) and put in a little more or less of a toner. We are talking about down to the tenths of a gram of color. Some toners are very strong, one drip could be a tenth of a gram or more. You mix your color on a very accurate scale, you pour in your toner up to a certain number, then pour the next up to the next number in the formula then the next and so on. With modern scales hooked to a computer there are some corrections that can be made. Older systems are simply pour up to the number and then change to the next toner in the "recipe". It is VERY easy to be off a hair. It just isn't that important most of the time to get them EXACT. For what ever reason, they are going to be blended anyway, they are going to be matched anyway, they are for an overall, etc.
Here is a great video on mixing on the scale. I just found it for this particular reason and didn't watch the whole thing so I can't support everything they say on it. But at about 6:50 they are mixing paint on a scale.
I just went and watched a little and right off the bat he calls the grams "Parts". insert rolling eyes smilie.
But if you are mixing say a five quarts of color (a gallon and a quart) for the customer, you mix a gallon in the gallon can, then you mix the quart in another quart can and hand it to the customer. Right off the bat if you added a drip or two more of a strong toner in both of them equally, the quart one is going to be changed MUCH more by that drip or two right? And being we are human, there is a very good chance the two are going to be different. How much different, often it doesn't even matter. But to be SURE, you intermix them.
The best way is to have them mix the quart in a gallon can. Then you just shake or stir them up well and pour a few quarts out of the gallon of paint into the gallon can with only one quart in it. Stir it up well and pour it back into the other gallon can, then stir that up and pour it back. Do this a few times and you KNOW you have the EXACT
same color in both cans.