That still wouldn't explain why Cortana acts so surprised when she discovers that Halo was built by an ancient alien race. The game makes is seem like it was her first to even learn about the forerunners existence when she should already be pretty familiar with them by studying the dig site on Reach.
Yeah, I'm gonna bump this, sorry for taking so long to get back to you.
No, Cortana would have been surprised when she saw the Halo because of a few simple things..
1) Cortana was in charge of the first ever Frontrunner artifact that humans had ever run into.. humans just did not know of anything else.
2) Cortana dug up information about a forerunner weapon that could turn the tide of the war.. she knew it was a forerunner weapon, she didn't know what it was.
3) The forerunner artifact on Reach was a) damage b) not even halfway done being examined.. this gave cortana very little time to learn about everything.. she hadn't seen anything on the Halo, other than it was a Forerunner weapon and it was important.. that is all.
so 4) She would have been surprised when she saw the Halo for the first time.. because it was the first time for her seeing it to. If you remember, she just told the pilot the coordinates.. she didn't know what she what they were going to come to either.