That's due to lag. The cast sequence waits with casting the next starfire until the previous starfire cast has finished casting on your client. When the previous cast has finished on the server (and your client hasn't detected it due to lag), your client will do a fallthrough and cast the nourish.

Try the following:
/castsequence starfire, starfire
/cast nourish

This will usually cast nourish after every starfire, even though there is no fallthrough.