The table separates out 1st and 2nd test takers, while your actual score report does not do that and shows everyone under one mean. That's probably the difference because 2nd time test takers included would definitely lower the 1st time taker's average. Look at 2012, 227 for 1st timers and 196 for 2nd timers. 196, even though the overall examinee # is much less, would definitely lower 227 considerably.
Above is the reason. There is not an error in the tables. There is an error in your interpretation of the tables. You were only reporting the 1st time test takers average, which is not accurate when you want to know the national mean for a year. You have to include ALL test takers, which those tables you provided do not do.
Therefore, the means and SDs listed on each individual score report that students get is the most accurate one. It may only be specific to their one test, but if you look back to the SDN Step 1 threads, it seems just about everyone usually has the same numbers listed in a given year.
Dermviser, come on bro. Didn't you match to derm, or have you not applied yet? A derm applicant should know how to interpret basic tables, considering a 12 year old would understand this.