Pāṇini lists the root śru in the bhū-ādi conjugation and then stipulates the rule śruvaḥ śr̥ ca [3.1.74] that changes śru > śr̥ and also prescribes the substitution of the infix/vikaraṇa śap > śnu.  Thus we get forms like śr̥ṇoti.  The form outwardly looks similar to sunoti, but the root su is in the 5th conjugation.  The question is why Pāṇini chose to list the root śru in the first conjugation and then go through the additional effort of prescribing the infix śnu as an exception to the first conjugation.  The commentary Tattvabodhinī on P.3.1.74 says the following: yadi ayam śrudhātuḥ svādau paṭhyeta tarhi cakāro na kartavya iti lāghavam ity āhuḥ.  So, the author of the Tattvabodhinī feels that Pāṇini should have listed the root śru in the 5th conjugation, and that would have been a more economical path.  I would like to hear an explanation as to why Pāṇini may have listed this root in the first conjugation.