Well, everything that the solution is in contact with allows heat from the solution to transfer to it , meaning the container gets warm too. The heat you calculated was only from the solution which is probably why the value is too low. Some of the heat flows to the container. Heat in the container = delta T x heat capacity of container. The heat capacity is usually given so you don't need to calculate it.
But if the lab instructions did not call for it, then perhaps the heat capacity of container was assumed low enough not to contribute significantly, I don't know.
Another problem might be some heat could be lost to the air escaping measurement.