I am reading Sipser's book and encountered a question with this regular grammer:
$$ R \Rightarrow XRX~|~S \\ S \Rightarrow aTa~|~bTa \\ T \Rightarrow XTX~|~X~|~\varepsilon \\ X \Rightarrow a~|~b $$
Apparently, the solutions in the back say that $T \Rightarrow aba$ is false. Why? Isn't this a derivation for $aba$?
$$ T \Rightarrow XTX \Rightarrow aTa \Rightarrow aXa \Rightarrow aba $$
On the other hand, $T \Rightarrow aba$, where the arrow has an asterisk on top of the arrow (I'm not sure how to make this symbol in LateX), is true. Why is there a difference?