How accurate is CC, really?

I've spent the last two years trying to lose weight on CC with disappointing results.  OK, I wasn't as committed as necessary, but even during the periods when I was perfect, I lost very little weight.

As a result, I decided to go on a Dr. supervised diet.  They've given me specific foods and portions that must be weighed.  According to them, these foods total X calories per day. 

I've just inputted a typical day into CC to check my nutrients and have discovered that what I'm eating totals 300 less than what the Dr. said it does! 

Given my previous experience with CC, I wonder how accurate the calorie counter is.  300 calories is significant amount to be off.

Any thoughts on the difference?

I think it's fairly accurate, but remember different people can add foods etc, and what they say could be wrong. Plus foods change over time!

I try and balance it out though, for instance I know that the bread I have is slightly more calories, so I work out how many extra grams I need to write to make it add up.

Fresh things I do just tend to trust though, but the problem is, 2 pieces of steak could be the same weight, but 1 have enough fat to add a few more calories, so again it has to be averaged.