The Real storage usage for IDMS will vary depending on what storage pages are needed at any given time.
IDMS does PAGE FIX a small amount of storage at the beginning of the IDMS region. RHDCOESA, SCAs, TCE/DCEs, etc. This is required as some of the routines and some of the control blocks these routines work with require them to be fixed. They cannot handle a page fault occurring.
To demonstrate. If you look at the Real storage for an IDMS system, especially one that is not very active (although this still works for an active system) and you issue DCMT DME STR C'sdughfsayrbjdjfh' (100) 100
(the Value for the STR is just something that will not be found.)
Then go back and look at the real storage you will see the number has jumped (increased) significantly. Eventually the OS will take the page back as needed and the real storage value will shrink.
This is not IDMS specific. This is standard OS paging rules.