Airlines can customise their 737NG displays to make them look like the older versions (a la Southwest Airlines)
Like This
While the standard setup is the new 777-style PFD/ND layout:
Like This
It is not a requirement however for the screens to look like the old type to allow a differences course, as easyJet have the new-style displays on their NGs which are flown by pilots who also fly the 733s.
My guess is that it's just customer preference.