The reason for the Varies is less that the program doesn’t check between Add and Set, and more that even the “Add” Fill option can vary. To explain what I mean, if you choose the “Add to budgeted amount” for a certain Envelope, while the screen will show a Fill amount of whatever your Envelope’s budgeted amount is at the time of creating / editing the Fill, when the Fill actually posts, it will calculate your budgeted amount at the time of the Fill, which can be different than when the Fill is created or edited. The Fill doesn’t attempt to divine the future, so it just says that the Fill amount may vary.
It is true there are certain cases that would indeed always have an explicitly fixed Fill amount, like if your Fill Adds a specific amount, rather than using a budgeted amount, but rather than run through a checklist to determine what case a Fill is, a scheduled Envelope Fill will simply report that its Fill amount “Varies”.
I hope that makes sense!