Analog Devices `SOIC_N` and `MSOP_N` Recommended Footprints
JEDEC publish dimensions for standard packages. Here is a list of dimensions for the narrow SOIC.

Individual component suppliers' published dimensions may differ from those slightly, depending on how they've implemented the tolerances.

However, the purpose of a board footprint, the solder pads, is to enable board assembly, so they will be slightly larger.

A large board manufacturer will usually optimise their footprints for their assembly process, which means tweaking the pads for their solder type (leaded or lead-free), solder volume (size of solder stencil aperture), heating rates, inspection methods, to get the best first time yield.

The component supplier's footprint, if they recommend one, is always a compromise between several assembly processes, so it's no surprise that different suppliers end up with different suggested pad layouts.

In general, the soldermask opening should be 'a bit' larger than the footprint of the package lead. This is to allow a visible solder fillet at the end and to the sides of the lead. This serves the purposes of making it possible to visually inspect the joint to check it's solder filled, to make it possible for surface tension forces in the molten solder to pull the package into alignment, and for increased area and solder volume to make a stronger joint. Make the pad too much wider, and you risk solder bridging between pads. 

The ideal extra width for pads (side fillet) will vary with lead pitch, and between machine and hand soldering. There is more lattitude on the extra pad length (toe fillet). It's useful to have them quite long to allow you a large place to land your soldering iron tip, and the only penalty for excess length is packing density, until you get into high RF where parasitics become important. You would usually also provide a heel fillet under the package. It can't be inspected, but provides extra solder volume and strength to the joint.

I tend to use a footprint with minimal side fillets, and generous heel and toe fillets, as I hand solder, and am frankly not very good at it, so need all the protection I can get from solder bridging.