Sean:
I don't remember Birkdale all that well, as it's been quite a long time since I was there, but it didn't seem that they were mowing the whole valley on several of the holes. The hole which I found most confounding was #6 ... even when they moved the tee up, guys would drive it past / over the bunker on the right only to watch it go into deep rough on the left (often on the flat) ... it seemed like the only play the set-up allowed was to lay up right to the corner, leaving you 240 to the green.