How does the melting point of alkynes change with the position of the triple bond?

$$\begin{array}{cc} \text{1-hexyne}&\pu{-132 ^\circ C}\\ \text{2-hexyne}&\pu{-89.5 ^\circ C}\\ \text{3-hexyne}&\pu{-103 ^\circ C}\\ \end{array}$$

These are the melting points of six carbon chain alkynes. The boiling point of a molecule is mainly affected by its shape. So, if all this molecules have the same shape, why is the range of temperatures so high ($\pu{42.5 ^\circ C}$)?

• Those molecules do NOT have the same shape, they look quite differently. – Karl Mar 4 '18 at 15:47