DT_HASH and DT_GNU_HASH produce different values

Question

I am reading the dynamic symbol table .dynsym and checking the DT_HASH and DT_GNU_HASH entries.

Oddly, I found out that the values are different. I know that DT_GNU_HASH can be a bit complicated to implement however to start I am simply using musl implementation found here.

/**
 * This is the code from Musl's dynamic linker.
 */
size_t gnu_hash_symtab_len_musl(const ElfW(Word) * base_address) {
  uint32_t nsym;
  const uint32_t *buckets;
  const uint32_t *hashval;
  buckets = reinterpret_cast<const uint32_t *>(
      base_address + 4 + (base_address[2] * sizeof(size_t) / 4));
  for (size_t i = nsym = 0; i < base_address[0]; i++) {
    if (buckets[i] > nsym) nsym = buckets[i];
  }
  if (nsym) {
    nsym -= base_address[1];
    hashval = buckets + base_address[0] + nsym;
    do nsym++;
    while (!(*hashval++ & 1));
  }
  return nsym;
}

Here is my DT_HASH implementation

      case (DT_HASH): {
        // https://flapenguin.me/elf-dt-hash
        struct hash_header {
          uint32_t nbucket;
          uint32_t nchain;
        };
        const hash_header *header =
            reinterpret_cast<const hash_header *>(base_address);
        sym_cnt_hash = header->nchain;
        break;
      }

For my shared libraries I am seeing two different values... For now it's not a big deal because I am taking the max but I can't understand why.

EDIT I found that adding the symoffset in the DT_GNU_HASH makes them the same. Those are symbols though are valid in my shared library and not just "STD_UNDEF" why are they in the offset?