If you have a project and need, say a 200 pf variable, and all you have is a 365 pf variable,
you can simply remove rotor plates one by one until your capacitance meter says you have achieved the correct value. Be careful, if you get close, generally that's just fine. A bit more is easier to deal with than trying to put a plate back on.
(And yes, you can even cut material off of one rotor plate to make it "exact".)
Now back in the early days of radio, the variables were all hand assembled and bolted together, so it was really easy to remove plates (or replace some if you goofed and removed too many plates.)
On the APC types it's easier to cut out individual sections of the stator plates most of the time.