It may be that the display models you see in the stores were not fitted with lamp covers or have had their lamp covers removed (for display purposes) - the bulbs that cookers take are all much the same - I'm sure they will all have covers when delivered new in their ex-factory condition.
But I could be wrong
If this is an issue for you - ask a sales person or just buy an oven that actually has a lamp cover.
Good luck...
Peccavi
January 2010