What is the reason behind Imam Ahmad's fatwa permitting the wiping of the head without wiping the ears, whether intentionally or unintentionally, even though the original principle of his madhhab stipulates the obligation of wiping the ears?

An Imam may have several different scholarly opinions (ijtihādāt) on a single issue. His followers first try to reconcile these statements. If reconciliation is impossible and the chronology is known, some will consider the last statement to be the madhhab (school of thought's official position), while others will consider the first. If the chronology is unknown, then the statement that conforms to the madhhab's principles is considered the madhhab. A distinction is made between the conventional madhhab (al-madhhab al-istilāḥī), which is what his followers chose and relied upon, and the personal madhhab (al-madhhab ash-shakhṣī), which is the Imam's direct text and personal choice. Wiping the ears during ablution (wuḍūʾ) is an example of this; there are two narrations from Imam Aḥmad regarding the obligation of wiping the ears, and his conventional madhhab is that wiping them is obligatory.