Can someone explain to me this thing I noticed at the military funeral of my grandpa's?

So my grandpa died a few months ago. He was a gp and during WWII he served in the Marines in the pacific. He was buried with full honors. Marines in dress blues. 3 volley salute. Taps. The whole town was there. I never felt more proud of being his grandsone. However I noticed something odd: He was a Marine. How... show more So my grandpa died a few months ago. He was a gp and during WWII he served in the Marines in the pacific. He was buried with full honors. Marines in dress blues. 3 volley salute. Taps. The whole town was there. I never felt more proud of being his grandsone. However I noticed something odd: He was a Marine. How come the pallbearers were from Marines,navy,airforce,army? I was expecting just Marines. This is just something I noticed that sticked in my mind and didn't really think much about it right on the spot but now thinking back.. well I wonder how come there were members from all branches to do that task. What do you think?
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