What most do not understand is that there were 4 days of fighting at Gettysburg, not just 3. On June 30th Pettigrews brigade advanced from Cashtown towards Gettysburg, for whatever reason is not known, but it certainly wasn't for shoes as is the popular myth. Anyway Pettygrew ran up against Buford's two brigades of cavalry and after a short shirmish Pettigrew withdrew back to cashtown. Thinking that they had only engaged a cavalry screen, Hill pushed forward the next morning, July 1st, a reconisence in force with his whole command to develope information as to the enemies strenght and movement, they being so close, and the rest is history.
The question will always be in the minds of arm chair generals as to why Lee stayed and fought at Gettysburg instead of falling back to his original plan, to Cashtown and South Mountain where his reserve (Longstreet's Corps) would be.
Of course Polley's account, as so many others, was written several years after the war and after Lee's death.