At the time of abu Bakr or even the Sahabah none of the 7 qira'at was established yet. Further the restrictions on the moshaf of 'Uthman or al-Moshaf al-Imam was not yet ordered. So reciting qur'an using synonyms based on the revelation and maybe even explanations was still allowed, as long as it was transmitted this way from the prophet ().
All of these 7 and more are based on the qira'at of multiple Sahabah . The earliest of these qira'at were established in the second hijri century. Some only in the third. Note that the so called 7 qira'at refer back to a choice of ibn Mujahid a student of imam at-Tabari (who had selected 8 qira'at in his book and did not consider Hafs 'an' Asim among them) which he compiled in a book, so they were only chosen in the fourth century about 300 years after Abu Bakr died.
Further at the time of Sahabah if any two of them had any difference in their recitation the qira'a wouldn't count as mutwatir. But this rather difficult to judge by now as we don't know of all of them.