Yes, you are missing something.
IF {list of conditions above} INCLUDING no mercury depositing in the hen or being excreted AND it all goes into the Eggs AND its all methylmercury (the only kind that matters for long term health concerns), that's 0.000000003 g of mercury for each gram of egg (average around 55g). The reality is lower still than these assumptions.
Again, dosage is TINY. MINISCULE. INFINITISMAL. Much smaller than dosage found in numerous consumer products. such as your tuna can.
FDA sets safe upper limit a 1 ppm for seafood. In other words, you would have to eat 330 eggs under the example (and assumptions) above to hit the safe limit for one egg's weight of seafood.
I submit to you that eating 330 eggs in very short order is likely to be unhealthy to you for reasons completely unrelated to potential mercury content.