In what setting did they apply? Alone/as a family/couple etc. That would make a difference?
Straight from the German embassy:
"Si el/la solicitante no tiene suficiente solvencia económica, debe traer original y copia de la declaración formal de compromiso (“Verpflichtungserklärung”). La firma del que invita debe ser legalizada por la autoridad de extranjería en Alemania (en original y una copia)."
Please keep in mind that the above is when the sponsor is already in Europe and for 90 day visa. This has never been the case with us, so we just both showed up at the embassy, me proving income, case closed.