If you look at several wood beams that came from the same location and they all look similar it’s likely because they all came of age in the same place and have saw marks or ax marks that were made at the same time.
If you sheared away the outer surfaces you might find the grains or inner color is different and you’ve got more variety of species than you thought. A quick and easy way to identify the species of a beam is to cut off the end to check the end grain, though this isn’t always totally reliable.
The exact species of reclaimed beams & timber gains importance when your intention is to shear away the rustic or original patina texture and get down to the grain for a smooth planed Craftsman style. This is when you want all your beams to be exactly the same wood species. But if the requirements are for Hand hewn or another Rustic texture, it’s often difficult to tell the wood differences.