This is often true of storage media. Doubly so for solid state storage. You are paying for more cutting edge, denser storage packages. Often the 500GB, 1TB and 2TB SSDs have the same number of storage chips. The higher storage tiers simply use bigger capacity packages.
Also, the higher capacity SSDs usually perform better than lower capacity. Though this is most apparent when jumping from 128 & 256 up to 512 and 1024 capacities.