Question about the taia library interface

According to http://cr.yp.to/libtai/tai.html, "A struct tai value is
an integer between 0 inclusive and 2^64 exclusive."  This makes sense.
For example, djbdns includes tai.h that defines struct tai as:

struct tai {
  uint64 x;
};

According to http://cr.yp.to/libtai/taia.html, "A struct taia value is
a number between 0 inclusive and 2^64 exclusive." I don't understand
this.
For example, djbdns includes taia.h that defines struct taia as:

struct taia {
  struct tai sec;
  unsigned long nano;
  unsigned long atto;
};


How can both statements be true if a struct taia contains a struct tai
within it?  Shouldn't a struct taia be defined to be "a number between
0 inclusive and 2^128 exclusive"? (or perhaps some other exponent..)