11 in modern Hebrew is "Echad-Esreh".
What is the etymological origin of the different word for 11 in the Torah (eg Shmos 36:14): "Ashtei-Esreh"?
My response was as follows:
I've seen an etymology of a contraction 'ad-shtei esrei, and absent other knowledge, this is attractive.
But I recently acquired Dr. Hayim ben Yosef Tawil's "An Akkadian Lexicon Companion for Biblical Hebrew", and he notes there that this is a cognate of Akkadian "ishten", meaning "one, unique, first". (He refers us to OA, OB on (CAD I/J 275a; AHw 400a), which is references on where to find it on cuneiform tablets, I think.) In Biblical Hebrew, he notes, it only occurs in combination with asar/esrei, meaning 'eleven'
In Akkadian (in CAD I/J 279b) there is the word ishteneshret, which is a cognate of the word eleven in Biblical Hebrew.
So, I suppose, it has no relation to shtayim, but due to historical linguistic accent, this word in this form got carried in to Biblical Hebrew, even though it did not in the general case.
I would note that he has no entry I can find for echad/achat, such that I guess Akkadian did not have such a word, but rather only 'ishten.