The spoken names of the opening letters have different structures. Some do not contain the combination needed for obligatory madd; others end in a normal long vowel; yet others are three-sound names with a madd or soft sound before a final still consonant.
«الف» is pronounced according to its own established name; not every three-sound naming pattern triggers the same obligatory prolongation.
«حا» and «طا» end in a long alif sound but lack a following original still consonant within their names.
«لام», «ميم», «قاف» and «نون» contain an internal long vowel followed by a still consonant.
Thus all disjointed letters cannot be assigned the same length. Pronounce each name correctly, examine its actual sound sequence, and then apply the rule appropriate to that structure.