That limitation as far as number of displayed changes was already noted in my document here:
R80+ Change Control: A Visual Guide
I'd say that all the necessary data is probably there in the postgres database to make the number of displayed changes reported on the policy installation screen only the ones that directly impact the configuration of that gateway. But what about things like Global Properties, Automatic NAT rules, implied rules and such? While it may well be possible to calculate this information, the overhead involved in larger configs would almost certainly cause a delay at the install policy screen and is probably not something that should be performed automatically every time policy is installed, unless the administrator explicitly asks for it. As it stands now the total number of changes being reported at the policy installation screen is quick and easy to calculate and just a basic sanity check.
In R77.30 and earlier SmartWorkflow had a "Compare Policies" (diff) feature that is exactly what you are looking for. There has been some occasional chatter about the functions of SmartWorkflow being integrated into R80+ management, see here:
Will (Smart)Workflow come back?
Supported software blades in R80
This was also mentioned in the follow up to the Dorit Dor AMA:
https://community.checkpoint.com/community/checkmates-live/blog/2017/09/02/i-am-dorit-dor-vp-of-prod...
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