When I used to brew with extracts, one of the more challenging things was to add the powder or syrup (more of a problem for the syrups) slowly enough to the kettle to avoid it from getting on the bottom and scorching. Usually, even if you can't taste any scorching or burnt sugar flavors, it will affect your SRM if it's happening to a lesser degree. The other thing, obviously, is that a person can't control the SRM as easily with extract since it's sort of pre-determined when it's made. That, and depending on how much water it's diluted into, will also affect the SRM of the final product.