curve25519 weak points for contributory behaviour
There are actually only 5 unique $x$-coordinates one needs to be concerned about:


$(0, \ldots)$
$(1, \ldots)$
$(-1, \ldots)$
$(x_1, \ldots)$
$(x_2, \ldots)$, 


where

$$\begin{eqnarray}
x_1 =&amp; 393823572354896145817230607815530211125 \\
     &amp; 29911719440698176882885853963445705823
\end{eqnarray}
$$
and
$$\begin{eqnarray}
x_2 =&amp; 32560625091655743179598362635611063129 \\
     &amp; 4008115727848805560023387167927233504
\end{eqnarray}.
$$

These correspond to the $x$-coordinates of points of small order on the curve and its twist, as you have pointed out. The remaining values you see are the multiples of those $x$-coordinates below $2^{256}$:


$0 \equiv 2^{255}-19 \equiv 2(2^{255}-19)  \pmod{2^{255}-19}$
$1 \equiv 2^{255}-19 + 1 \equiv 2(2^{255}-19) + 1 \pmod{2^{255}-19}$  
$2^{255} - 19 - 1 \equiv 2(2^{255}-19) - 1 \pmod{2^{255}-19}$  
$x_1 \equiv 2^{255} - 19 + x_1 \pmod{2^{255}-19}$
$x_2 \equiv 2^{255} - 19 + x_2 \pmod{2^{255}-19}$


These extra values are checked because curve25519 was designed to accept any $256$-bit string as public-key, and each value has one or more 256-bit representations modulo $2^{255}-19$.