They did not use the exact same mRNA sequence: https://github.com/NAalytics/Assemblies-of-putative-SARS-CoV2-spike-encoding-mRNA-sequences-for-vaccines-BNT-162b

They did not use the exact same mRNA sequence: https://github.com/NAalytics/Assemblies-of-putative-SARS-CoV2-spike-encoding-mRNA-sequences-for-vaccines-BNT-162b2-and-mRNA-1273/blob/main/Assemblies%20of%20putative%20SARS-CoV2-spike-encoding%20mRNA%20sequences%20for%20vaccines%20BNT-162b2%20and%20mRNA-1273.docx.pdf https://github.com/NAalytics/Assemblies-of-putative-SARS-CoV... The different doses were guesses made to balance effectiveness with severity of side effects. (Earlier Moderna trials, at least, had included even higher doses, which could cause pretty severe side effects.) Their choices had to be locked in early, based on very limited preliminary trial data, in order to run their phase 3 trials as quickly as possible, to actually bring the vaccines to market quickly in 2020. They just made somewhat different bets. I think it's as simple as that.