Why in Parshas Bo does it say:

וּמַצּוֹת עַל מְרֹרִים יֹאכְלֻהוּ

But by the Parsha of Pesach Sheni in Bamidbar(Tes,Aleph) it says:

עַל מַצּוֹת וּמְרֹרִים

Why the Change in the Language?