Ceres overprint

Please can anyone tell me why the 1938-40 Ceres issues were overprinted in red with 1F which seems to have applied regardless of the original value.
Apart from catalogue mentioning of this I cannot find an explanation in terms of changes to postage rates in any of my reference books but, I guess, it was due to the war.
Why did the Ceres issue not have the original value cancelled by two bars as were other issues at the time? I assume it was because the values were so small on the stamps.

The 1f75 Cérès was issued to meet the foreign letter rate (1 Aug 1937-30 Nov 1938)and hence printed in blue (UPU regulations). Similarly the 2f25 Cérès was issued to meet the next foreign letter rate (1 Dec 1938-31 Dec 1939) and was likewise printed in the regulatory blue. The 2f50 Cérès was issued to meet the Internal Registered Letter Rate (17 Nov 1938-30 Nov 1939).

When the next round of rate changes took place(1 December 1939 for internal mail and 1 Jan 1940 for foreign mail)the above three stamps became redundant either because of their value or because of their colour. At the time there was a considerable paper shortage (hence the number of slogan postmarks advocating paper economy measures)and so the stamps could not be destroyed. Instead all the remainder stocks were overprinted with a bold 1f overprint which met the new basic internal letter rate (1 Dec 1939-4 January 1942). It is estimated that over 35,000,000 copies of these three values were recycled in this way.

I do not know why the original value was not cancelled. I assume the large overprint was sufficient to indicate the revised value.

Thank you very much Mick, I am most grateful for both your replies.