Quite possibly that some places had extra stock that they wanted to
use up. That's the only thing I can think of,

I would agree.
Here is the explanation between postage and revenue use
of Victorian stamps by Stanley.

Yours being printed at the true "postage or revenue" period.

THE "POSTAGE AND REVENUE" PERIOD, 1884-1901

Under the provisions of the Postage Act 1883 the stamps of the three series
then in use (Postage, Duty, Fee) became, as from 1.1.84, mutually
interchangeable. It was, at the same time, decided to issue (as soon as
possible) the one stamp only, for any value, to serve all purposes. Since
there were available many more dies (and plates) inscribed "Stamp Duty" than
there were of either the "Postage" or "Fee" (Stamp Statute) series it was
agreed that all values should be inscribed "Stamp Duty" by the beginning of
1885. All stamps printed after 1.1.84 are therefore true "Postage and
Revenue" stamps whereas all Stamp Duty and Fee stamps printed before that
date are Postal Fiscals, since they were originally printed solely for
fiscal purposes. These principles have been strictly adhered to in our
listing. Little difficulty should however be met in distinguishing between
the printings of the one stamp found respectively in the main list and in
the "Postal Fiscal" section since there are many major differences of
printing, watermark, perforation and shade. On 1.1.84 there were no "Stamp
Duty" designs for the Vad., 2d., 4d., 8d. and 2s. 6d. values. Also the
existing "Stamp Duty" designs for the Id., 6d., Is. and 2s. were deemed to
be too large to be convenient for general and extensive use. For all these
values it was therefore necessary to produce new and smaller designs
inscribed "Stamp Duty". Pending the preparation of new dies and plates,
printings were made in 1884 (for the Vzd., Id., 2d., 4d., 6d., Is. and 2s.
values) from the existing "Postage" plates. These printings are also
"Postage and Revenue" stamps but have naturally been included, for the sake
of convenience, in the previous period. By the beginning of 1885 printings
were available, in the new designs, of all values save the Is. and 2s., and
these latter appeared later.