פרשת וארא - To...

ח:כא וַיִּקְרָא פַרְעה אֶל משֶׁה וּלְאַהֲרן

Isn't it strange that the prefix for Moshe is אֶל whereas that for Aharon is לְ?

I think the answer lies in the :גמרא יומא ד where it is stated "קול לו קול אליו - משה שמע וכל ישראל לא שמעו" What the gemara is ultimately saying there is that the term אליו connotes a more direct and focused manner of speech than does לו. Perhaps here, the call of Paroah was direct primarily at Moshe. The call for Aharon was slightly less significant and so, the term אֶל is used for Moshe but not for Aharon.