That’s because Delta-9 THC, the compound we have always known as plain ol’ THC, begins as THC-A.
There are two forms of what is called ‘hemp’: industrial hemp, which has been bred for things like fibre, and the “legal classification that is hemp”.

The legal classification varies depending on region, however it requires the plant to have a very low amount of THC (usually something around 0.2%).

The farm bill added various loopholes, which allowed for high THC-A and low THC samples to pass legally as hemp. Though it is still cannabis and always has been, the plant has just been tested long before the harvest and before the THC-A has had time to convert to THC.
Once the weed packs arrive at the shop, I guarantee to you that enough THC-A has converted to THC that it no longer would be classed as hemp. I love the hilarity of it all.