Presumably because it's less concrete. If you häller nykokt sylt i en burk then that's one individual jar that you fill up and you describe it in very concrete terms, perhaps not even intending to store the jam. But if you talk about the method, or about working according to the method, then it's indeed häller den nykokta sylten på burk. Or: tappar ölet på flaska, lagrar whiskey på fat.
Not sure whether this would be the standard explanation, but the only other explanation I can offer is that this is just how it's said when it comes to food preservation (but not in other cases, such as kaffe i muggen eller vatten i kastrullen).