This answer is also simple. This is due to how much terpenes are in the vape cartridge, and what cannabis terpenes are used in general. Some terpenes are stronger and overpowering than others, so companies tend to use less of that terpene. This means the vape cartridge will likely be thicker.
Some terpenes are a little weaker and need more, therefore making the Delta 8 THC in the cartridge a little more runny. Having vape carts that are too runny is a bad thing, as this means way more terpenes than Delta 8. Solid Delta 8 vape carts mean there is a lot of Delta 8 and not a lot of terpenes, making it more potent.
Each strain will differ for this reason. For example, our Binoid Delta 8 THC Vape Cartridges are 92% Delta 8 THC, with 3-4% terpenes and 4% plant lipids. This is the optimal amount for us. Some brands only have 70% Delta 8 THC. Usually the higher the better, and anything above 90% Delta 8 THC is really good.

