I have noticed that there are two types of 757-200s. They are mainly distiguishable by the location of their emergency exit doors. The type with the two small doors over the wing are operated by AA, UA, DL, CO and I think TW. The type with one door BEHIND the wing is operated by US, HP, N7 and BA. I have noticed that NW operates both types and yet they are all called the 757-251. Why are there two types?