I've noticed while using Altium that SOIC ICs have 3 different SOIC PCB layout to select from, they are named NSOBX_L, NSOBX_N and NSOBX_M (being X de number of pins). The difference I've noticed is that the 'L' variant has smaller body size, and all of them have the same distance between pads but pad length varies through all of them. I know that when deciding which one to use the wise thing to do is checking if they fit the manufacture's recommended layout for the specific IC but I'd like to know anyway.
EDIT: I've checked the PCB layout and confirmed that lead pitch is 1.27 for all of them. From the wikipedia link that Andy left in the comments I can tell that MSOIC is excluded, there's a chance that the 'N' SOIC variant stands for Narrow SOIC but still there are the other two left.