DMN allows two styles for listing the input data required by a decision.
The original style is to attach an Input Data shape to the Decision shape and this is the style Signavio uses.
However, when a Decision requires a lot of inputs, this clutters the diagram.
The other style is to list the input data items as line items within the Decision shape itself, which produces a larger Decision shape but a less cluttered diagram.
The attached diagrams illustrate the same Decision model using both styles. I hope people can see that the second style is much neater (and this is not a particularly complex decision model).
Please allow the user to choose between styles when creating Decision diagrams in Signavio.