Tim,
The figures reported by jurisdiction in Annuario Pontificio are those provided by the jurisdiction and, for those in the US, (should) correspond to the data in the Official Catholic Directory of the US, published annually by PJKenedy & Sons (if I get an opportunity tomorrow, I'll compare a prior year in the OCD to the same year in the AP for a couple of jurisdictions, just to verify that there are no differences). That said, Stuart makes valid points about the factors that artificially inflate numbers.
As well, there are just plain outright errors - some of which get corrected each year, some of which are created each year.
Witness, for example (overlooked in our discussions on the other thread, because we were focused on the US jurisdictions), the Slovak GCC data for the Eparchy of Bratislavia 2009-484,046 versus 2010-20,000

Similarly:
the Ukrainian Exarchate for Great Britain and that for Germany & Scandanavia reported decreases in the number of faithful in that same period of 40,000 and 20,000, respectively (probably getting closer to actual figures), while the Exarchate of Odesa-Krym would have us accept that its numbers increased by 18,000 and the Archeparchy of Lviv by 85,000 (with no corresponding decreases anywhere else);
the Armenians report a drop of 100,000 in the figures for their East European Ordinariate - undoubtedly appropriately;
the Maronites, however, claim a one year gain of 39,000 in the Archeparchy of Tripoli and of 32,000 in the Patriarchal Eparchy of Joubbe, Sarba and Jounieh;
the Chaldeans report an overall increase of 45,000 faithful over that same single year period;
the Malabarese claim to have had an increase of 14,000 in the Metropolitan Archeparchy of Trichur;
the Melkites overall increase is only 11,000, but the data would have us to understand and believe that all but 1,000 of those are attributable to the Australian Eparchy and the Archeparchy of Saida in Lebanon, each claiming 5,000 new faithful;
even the tiny Exarchate of Athens for the Greeks wants to claim an addition of 200 faithful, a 9% increase.
The only number that jumps out at me as likely very accurate is that of Grottoferrata - the Abbey reports a loss of 1 - from 88 to 87.
If anyone believes the increased numbers that I cited to be accurate, I have a cathedral in Antarctica that I'll sell very reasonably - cashier's check only, please.
Many years,
Neil