Fermentables and non-fermentables in liquid malt extract

Is it possible that some extract may have a higher percentage of non-fermentable sugars, resulting in a sweeter but less alcoholic beer than orginally planned?

Just wondering as I brewed two seperate beers which both did not attenuate as much as I (and BeerSmith) had expected. Is this because of the extract or just a stuck fermentation?

Yes, different extracts have different amount of fermentables. Most are around 75% fermentable (with the average yeast), but something like Laagerlander is only 55% fermentable.

BUT ... you probably want to look more at your yeast and processes. Some yeasts have low attenuation (example; Windsor). Lack of aeration will reduce yeast cell growth resulting in low attenuation. Very low pitch rates can stress out the yeast and result in low attenuation. Low temps can stall yeast. Etc, etc, etc.