All files delivered as part of CP1 are also delievered in CP2 and all files in CP2 are included in CP3.
So even if a file doesn't change in between, it still has to be part of latest cumulative. Ideally if a file is not changed, it should have same timestamp. This is followed in TFixes. But this is not the case with cumulative fixes because of a different build process.
I can request a change log to be included in future cumulative patches that can tell what deliverable files have changed between two cumulative patches.
Regards,
Imran