A .45 ACP case has a SAAMI maximum length of .848".
I think you mis-typed. the .45acp max case length is .898"
I understand. and you are correct, that loading shorter bullets to the listed 1.275" does change the internal volume of the loaded round.
Now, there's two questions, (and specific to the .45acp), first why would you load to 1.275" if its not needed for function, and second, if you do, does the change in volume MATTER??
Something I'm seeing very often is people who seem to think they have to load to the listed COAL. They don't seem to understand that the max COAL is a limit, not a requirement. A lot of them also seem to think that SAAMI pressure limits are a "your gun will grenade if you pass this by any amount" limit, which, of course, they aren't.
What is the "right length" for a load?? There are SAAMI max length limits, there are given lengths for some bullets from their makers, and then there's what
will work in your gun, and what works best in your gun (and those can be different).
Another point many don't seem aware of is that different rounds, and different guns don't always follow the same exact formulas for what works and what doesn't. Generally similar, but seldom identical, and that all loading data are guidelines not laws of nature that don't vary in the known universe.
You know those instructions on the box of whatever you're cooking, giving temp settings and times (25min @ 425degrees, etc.,) but ALSO say ovens vary so cooking times may vary....
(and then, there's also altitude variances...)
Guns vary, too.