The reason is that keys are not the same as addresses, and it is not a one-to-one mapping of pubkey to address. There are multiple addresses for a single pubkey as there are multiple address types. The purpose of descriptors is to make it clear which address type to create. However you've completely bypassed that by taking the key out of the descriptor and trying to compute the address manually. The reason you don't get the same address is most likely because you have chosen the wrong address type.