Some people have favorite submitters, and so well established accounts may get more readers, and hence more upvotes, than newer accounts. Your question included the proviso that both submissions got the same number of upvotes in the same time - that might not happen with accounts that have difference karma.
And not all upvotes are equal. The software has voting ring detection that is good, but not perfect. Submissions often get quite a lot of upvotes that don't convert to karma for the user simply because not all upvotes are regarded as "genuine". Those upvotes might not count towards the submissions ranking, but I don't know.
What I do know is that there is "secret sauce" and that it's generally pretty good, although not perfect. Trying to game the system is rarely successful, and trying to understand it from the outside is going to be really hard.
Personally, I submit stuff I think is going to be of interest, I read stuff that looks like it might be interesting, and then I walk away. The internet in general has taught me not to care.