Each overprint to revalue a postal item (whether up or down) must be evaluated on its own merits and should not be broadly painted as inflationary or deflationary.
Many postal stationery items are revalued to use up obsolete stock when the rate changes. Here is a U.S. example following the 1919 rate change (and 3 cent government-stamped envelopes of this period show a similar 2 cent revaluing overprint.)
Similarly to use up obsolete stock, in the early 1950s, the 1 cent Jefferson card (and others) were up-rated to 2 cents.
Stamps have the same variety of individual explanations, especially in remote countries supplied by the motherland. For instance, they run out of 2 cent stamps. What do they do? They have plenty of 5 cent stamps, so they revalue them as a matter of supply/demand to solve a local problem. Completely unrelated to inflation/deflation.
Or a currency change, etc. etc.
Vayolene, What was the reason for the French value-change?