[…] never imagining some lotteryWill change her load of pottery to wings. (12-13)
If, throughout your life, people in authority keep telling you that you're unworthy, it's hard not to believe it yourself. In line 13, the word "pottery" recalls the earlier reference to "serving dish," connoting even more humble dinnerware. This female turtle doesn't even dare imagine that she has the capacity to rise above the limiting circumstances of her life.
Her only levity is patience,The sport of truly chastened things. (14-15)
"Chastened" can mean "subdued," "repressed," or "punished." These meanings are certainly relevant to bias against women. And one synonym for "patience" is "endurance." Throughout history, women have suffered physical, emotional, and economic subjugation. Their ability not only to endure, but also, over time, to progress in their struggle for fundamental human rights is cause for celebration, don't you think? Maybe the little glimmer of "levity" and "sport" in the poem acknowledges this hope for a brighter future.