Most resin kits are filled at the manufacturing plant by weight. With some types of resin, it may look like one bottle has less volume than it should. Typically you are often actually getting more product than advertised even if the liquid volume looks lower in the bottles.
Some epoxy Resin bottles are filled by weight, not volume (so not fluid ounces). With some types of epoxy resins, the Part A Resin is a much heavier weight, then the Part B Hardener . Sometimes as the bottle sizes get bigger, it becomes more noticeable a variance, due to the bottle of the heavier liquid expanding/bulging ever so slightly on the sides. When you put a calliper on the part A bottle, (of this 2 gallon kit shown for example), it’s actually a half an inch wider. The viscosity & heavier weight of the resin in Part A, has caused the bottle to bulge sideways a bit, which gives the appearance of less volume. But if you remove the vacuum seal, and then you squeeze the bottle a half inch (so caliper is same width on both bottles), the liquid will come up to meet a similar level of Part B.
There is usually slightly more product than the advertised amount in the bottles. Note: A US gallon = 3.785 liters (or an Imperial Gallon = 4.546 liters, or the commonly accepted "metric" gallon equals 5 liters), but since these are made in USA, it is based on the US gallon measurements.
Here are a few examples to show that the weights poured in a bottle, give more then ounces stated:
64 oz kit, is 4 lb, 10 oz. That translates to about 4.5 lbs. But when you do the math, 4 .5 lbs is 72 oz. The warehouse is really giving us 1/2 lb more product than what we advertise.
A gallon is 8.34oz, each of these bottles each have 8.56oz. Again, you are getting slightly more then 2 gallons in a kit.
The one and a half gallon of a 2:1 ratio thinner lighter viscosity resin, may weigh 12.9 lbs. However, 12 lbs is exactly 192 oz. (192 oz = 1.5 gallons), again the bottles may contain slightly over amount stated.
Sometimes shipments come in with the bottles looking more even & full, because occasionally the warehouse, will top up even more over the (paid for) weight.
Also note, the resin bottles are not typically filled to the top, because the liquid needs room for expansion. Smaller bottles are not used because of the need for the expansion room, & there’s far less issues with shipping leakage when not full to the top. Also on the rare occasion they were filled full to the top, they often leaked all over the edges upon opening, creating an unexpected mess all over the bottles.
Another more recent possibility for bottles showing more of a difference in levels, (during the recent shortage of bottle supplies), plastic bottle manufacturers may be different from batch to batch, so one type of plastic bottle may hold the weight more evenly throughout, then another type of plastic bottle (which may effect slightly more bulging in the heavier liquid bottle).