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Upekkhā word is used in 10 contexts of suttanta with 10 meaning, which were analysed by sāriputta-aggasāvaka, then concluded to 4 sabhāva:
So, when sāriputta try to explain each upekkhā, he used the synonym words to explain them, like we saw in the dictionary, nowadays, such as in KN. Jūlaniddesa, Udayamāṇavakapañhāniddeso:
[446] Upekkhāsatisaṃsuddhanti upekkhāti yā catutthajjhāne upekkhā upekkhanā ajjhupekkhanā cittasamatho cittappasādatā majjhattatā cittassa.
That is the original tatramajjhattatā in pāli cannon.
Because of the main topic of each contexts, which each upekkhā word appearing. For the example in SN Mahāvagga, First Ānandasutta:
[1393] Yasmiṃ samaye ānanda bhikkhu tathā samāhitaṃ cittaṃ sādhukaṃ ajjhupekkhitā hoti upekkhāsambojjhaṅgo tasmiṃ samaye bhikkhuno āraddho hoti upekkhāsambojjhaṅgaṃ tasmiṃ samaye bhikkhu Bhāveti upekkhāsambojjhaṅgo tasmiṃ samaye bhikkhuno bhāvanāpāripūriṃ gacchati.
You can see sama-word and ajjhupekkhā-word, which appeared in Udayamāṇavakapañhāniddeso like tataramajjhupekkhā, explaining upekkhāsambojjhaṅgaṃ. So, commentary concluded them as the same type upekkhā, tattaramajjhupekkhā.