Calculating ΔH with non-integer coefficients in the balanced equation
There is not much to clarify: you determined dimolar enthalpy of combustion, whereas standard enthalpy of combustion is determined for a complete oxidation of one mole of the substance:

$$
\begin{align}
\ce{C2H6(g) + 7/2 O2(g) &amp;-&gt; 2 CO2(g) + 3 H2O(l)} &amp;\quad &amp;Δ_\mathrm{c}H^\circ\\
\ce{2 C2H6(g) + 7 O2(g) &amp;-&gt; 4 CO2(g) + 6 H2O(l)} &amp;\quad &amp;Δ_\mathrm{c}H^\circ ×2
\end{align}
$$

When composing the corresponding reaction equation, make sure to balance it in such a way that the stoichiometric number $ν$ of the substance the enthalpy is determined for is always $-1.$