There are two places in the Torah that I can recall (if there are others, please inform me) where the term עַשְׁתֵּֽי־עָשָׂ֥ר is used for the number 11.
בְּיוֹם֙ עַשְׁתֵּ֣י עָשָׂ֣ר י֔וֹם נָשִׂ֖יא לִבְנֵ֣י אָשֵׁ֑ר פַּגְעִיאֵ֖ל בֶּן־עָכְרָֽן׃
On the eleventh day Pagiel the son of Ochran, prince of the children of Asher:
וַיְהִי֙ בְּאַרְבָּעִ֣ים שָׁנָ֔ה בְּעַשְׁתֵּֽי־עָשָׂ֥ר חֹ֖דֶשׁ בְּאֶחָ֣ד לַחֹ֑דֶשׁ דִּבֶּ֤ר מֹשֶׁה֙ אֶל־בְּנֵ֣י יִשְׂרָאֵ֔ל כְּ֠כֹל אֲשֶׁ֨ר צִוָּ֧ה יְהוָ֛ה אֹת֖וֹ אֲלֵהֶֽם׃
And it came to pass in the fortieth year, in the eleventh month, on the first day of the month, that Moses spoke unto the children of Israel, according unto all that the LORD had given him in commandment unto them.
Why does the Torah use this term instead of אַחַ֨ד עָשָׂ֥ר? Is there some unique message in these two places that is being implied with the use of this term, or is it somehow related to its grammatical placement or usage?