In Shemot 12:34 we are told that Bnei Yisrael left Mitzrayim מִשְׁאֲרֹתָ֛ם צְרֻרֹ֥ת בְּשִׂמְלֹתָ֖ם עַל־שִׁכְמָֽם Older translations followed Ibn Ezra and Targum, that they left Mitzrayim with their 'kneading troughs wrapped in their clothing, [carried] on their shoulders.' Rashi there, however, defines מִשְׁאֲרֹתָ֛ם as 'leftovers,' which Artscroll, Aryeh Kaplan etc follow, and there is the takeaway that, as Rabbi Frand put it, you don't pack what you really value in checked luggage; it stays with you as carry-on.
All fine and good.
But back in Shemot 7:28, when the frogs are invading every nook and cranny of Mitzrayim, that verse uses the same word וּבְמִשְׁאֲרוֹתֶֽיךָ and this time Rashi is silent.
Everyone seems to accept the kneading trough idea at 7:28. What led Rashi to offer a very different interpretation for 12:34?