Possible writting fail on Godzilla 2014 (Possible spoilers!)

After finnaly getting my hands on a decent quality pirated copy of Godzilla 2014, Upon a second vewing, I noticed something that left me with a "wut?" face.

When Joe and Sandra enter the jeep to go to the nuclear plant, they say goodbye to Ford that boards the schoolbus and the following dialog occurs:

-He made you a sign-says Sandra

-Huh?-says Joe

-Your birthday sign. He worked so hard. I will pick him up and then we can get a proper cake.

Joe is a bit annoyed with himself for not noticing that and changes subject to the strange readings that he is getting. Suddenly this happens:

-Happy Birthday!-says Sandra

-What? Is it...!?-says Joe surprised

-Yes it is!-and Sandra hugs him.

Yup. Unless I got this wrong (if so, tell me) and they were pretending already that he did not know about the birthday,  then there was a logic gap here.