Why return s._hash[3];？

[UMU618]

fc/include/fc/crypto/sha256.hpp

Line 131 in 0784e1b

return s._hash[3];//*((size_t*)&s);

Why not return s._hash[0];?

[gzliudan]

_hash[3] is lowest uint64 in uint256 which has 4 uint64