Upekkhā word is used in 10 contexts of suttanta with 10 meaning, which were analysed by sāriputta-aggasāvaka, then concluded to 4 sabhāva:
- One refers to vedanā (upekkhā-feeling).
- One refers to viriya (effort).
- Two refer to paññā (understanding).
- Six refer to tatramajjhattatā (equality/parity/counterbalance).
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:
 Upekkhāsatisaṃsuddhanti upekkhāti yā catutthajjhāne
upekkhā upekkhanā ajjhupekkhanā cittasamatho cittappasādatā majjhattatā cittassa.
That is the original tatramajjhattatā in pāli cannon.
Why commentary concluded 10 meaning to 4 sabhāva?
Because of the main topic of each contexts, which each upekkhā word appearing. For the example in SN Mahāvagga, First Ānandasutta:
 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
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.