SSOP and SOIC standards
It's not because 0.635 and 0.65 are close to each other that they're meant to be the same. There are also 2.5 mm pitch connectors next to the more standard 0.1" = 2.54 mm. Metric vs imperial. They're just different things which should be treated as such.

Note that with packages going further away from DIP the imperial standard is often abandoned in favor of metric. For instance DFN and QFN are mostly 0.5 mm pitch, and other dimensions are in mm as well, like in this drawing. Dimensions for SMD in inches becomes rare.