
Makes sense.

Think of it like this.
Two main genders: Female and Male
But there are three subgenders: Alpha, Beta, Omega
Here are your possible combinations
Female Alpha
Male Alpha
Female Beta
Male Beta
Female Omega
Male Omega
Alpha's are considered the top of the pyramid due to their charismatic and superior ability fertility (Idk sperm donor?). Distinguished by a knot at the base of their penis that is used to ensure pregnancy.
Their most compatible partner are usually Omegas because they often to give birth to more Alphas and Omegas
Betas are the middle and the average lot. Most of the time they give birth to Betas but there are rare chances to have Omega and Alpha. Basically average fertility and your average person.
Last but not least, Omegas are considered the bottom of the social pole due to their higher ability to get pregnant (In crude terms they are breeders). They don't usually have a high rate of producing children but they can conceive better than Betas. Distinguished by their heats that happens for a week every number of months depending on the author.
That's the gist of it. The Alpha and Omega thing is more of a gender rather than who tops and who bottoms.
I hope this helps

This also might explain a little better than what I just told you.
http://www.mangago.zone/read-manga/kashikomarimashita_destiny/mi/f934afad5dcaafc116852fb7793fd141_chapter-1/6/
On one of the pages it says it's Omega Eren x Alpha Levi..But shouldn't it say Omega Levi x Alpha Eren?