"housekeeping genes" in bacteria are commonly expressed constitutively, but not all of these genes are expressed at the same level (the same number of molecules per cell). the primary mechanism responsible for variations in the level of constitutive enzymes from different genes is that:

Housekeeping genes may be defined as the genes that are constitutive genes important for the cellular functioning of the organism. The expression of housekeeping gene may or may not vary.

Some housekeeping genes are not expressed at same level because the housekeeping genes also have different promoters. The different promoters have different affinity to interact with the holoenzyme of RNA polymerase.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: