why not ' nous avons vu' rather than 'on a vu' ?
I guess it has not been marked wrong, we can say both.
Both are acceptable. Nous always means we, whereas on can mean we, you & (some)one. Context will tell you about the latter. But when you translate "we", use either one. Just remember to match the verb(s).
When is it preferrable to use "on" rather than "nous"
On is less formal than nous. Use on in everyday speech and casual writing.