In the case of Slim and others, you are getting involvement in the war substantive rank system, with its temporary, local acting, acting, substantive and war substantive ranks.
In the RAF the affect of this can be seen in the post-war regrading and rebadging of NCO aircrew and the wholesale reassessment of officers' rank, the former in 1946 and the latter in 1947 IIRC.
If you can find anybody who can explain the system and its funny bits in less than 10000 words, please put them in touch.
Finally, I was led to believe that 'Acting Pilot Officer' is actually a substantive rank - someone will quickly put me right if that's wrong!!
Old Duffer