but the results display fine on screen.
This means report view?
If so, then I believe the answer to your musing is that in this view, the entire report is "available" to itself thus calculations can work but might not be when it's formatted for printing. Print Preview is just basically that, and whatever you see on screen should be the paper output, so if you're saying that this error is raised in either of these cases, then a calculated control's control source may not be available in preview. The entire report is apparently not "available" to itself. Not the best way to describe it I guess. You could try moving the control to a header or footer (page, report, or section). You didn't say where it was located.
I also advocate doing as much of the calculation in a query, but sometimes it's just easier/better in a report - especially if it's a calculation over a group.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.