The difference is likely vacuum actuation points and/or metered flow rates.
Similar to the variances seen in vacuum advance canisters, different combinations required different EGR valves to reintroduce the proper amount of spent exhaust gas at the right time of operational load.
For instance, according to the 1976 Pontiac Engine Assembly Manual, Pontiac used a total of (4) different EGR valves for 400 4-barrel applications and (1) EGR for 400 2-barrel applications.
And if anyone is counting, Pontiac used (6) different EGR valves for 455 4-barrel applications that year--only one of which is shared with the 400 4-barrel applications.
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