Continuity error.. Becky's singing

I can think of at least two episodes (The telethon 90 one and Becky and Jesse's wedding) where Becky is a decent singer. So the episode where she wants to sing to the twins before bed time and it turns out her voice is like nails on a chalkboard makes no sense. Don't get why they did this, as if no one would ever remember she had sung decently in other episodes.