>  We don't know the persons responsible for the data,

Sorry. I should have supplied that information. This was an official US Government report from 1918

Jones, S. A. (1918). Honeybees and honey production in the United States (No. 685). US Dept. of Agriculture.

Here is a bit more info from it. 

EXTENT OF INDUSTRY.

The importance of the honey industry in the United States
is realized by few, even of those who have given some attention
to the subject, owing to the fact that the census figures,
which are naturally looked to for information, report only
concerning the bees belonging to farmers, whereas a large
and increasing proportion of the honeybees of the country,
producing a still larger proportion of the honey crop, belong
to beekeepers other than farmers and are located in villages
and towns, or, by permission of the farmer or landowner,
are kept in out-of-the-way places on farms or hidden in the
recesses of the hills, away from frequented roads where they
would be readily observed. Surveys of the beekeeping industry
in Massachusetts and Indiana indicate that the census
included hardly more than half the actual number of colonies
of bees in those States and checks on honey production show
similar deficiencies in other States.

The first census to make inquiry concerning the number
of colonies of bees on farms in the United States was the
twelfth, which reports the number of colonies belonging to
farmers on June 1, 1900, to be 4,108,239. The next census
(1910) reported 3,445,006 on hand as of April 15, 1910, but
the last showing is unfairly low even compared with the
first, because the date of the last census is 45 days earlier
than the former and at a period of the year when the number
of colonies is increasing rapidly from swarming. 

In the greatest of the honey years recorded by the census,
1889, the figures for some States are quite astonishing from
the standards of other years, New York producing over
4,000,000, Illinois and Missouri between 4,000,000 and
5,000,000, and Iowa almost 7,000,000 pounds, this being
for New York a third more, for Illinois and Missouri in
excess of a hali more, and for Iowa more than double, the
crop of any other year reported.

* * * 

The fact that bee diseases drive out of
business the untrained and indifferent beekeeper, who is
unable to successfully combat them, opens the field to the
man capable of overcoming them, who is usually also a
deeper student of the entire subject of bees and of their care
and protection, better informed on sources of nectar, dates
of flow, etc., and is therefore able to handle the bees with a
view to maximum honey production.

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