I'm sorry for your loss. My best friend died in January while on active duty. He received the three volleys, which is often confused as the gun salute.
The only reason I can think of why your grandfather did not receive the three volley honor is that perhaps the funeral detail was overbooked and could not support it.
From another site:
Graveside military honors include the firing of three volleys each by seven service members. This commonly is confused with an entirely separate honor, the 21-gun salute. But the number of individual gun firings in both honors evolved the same way. The three volleys came from an old battlefield custom. The two warring sides would cease hostilities to clear their dead from the battlefield, and the firing of three volleys meant that the dead had been properly cared for and the side was ready to resume the battle.
Arlington National Cemetery follows an "order of arms" protocol to determine the number of guns to be used in a salute. A president, ex-president or foreign head of state is saluted with 21 guns. A vice president, prime minister, secretary of defense or secretary of the Army receives a 19-gun salute. Flag officers receive salutes of 11 to 17 guns, depending on their rank. The rounds are fired one at a time.
A U.S. presidential death also involves other ceremonial gun salutes and military traditions. On the day after the death of the president, a former president or president-elect -- unless this day falls on a Sunday or holiday, in which case the honor will rendered the following day -- the commanders of Army installations with the necessary personnel and material traditionally order that one gun be fired every half hour, beginning at reveille and ending at retreat.
On the day of burial, a 21-minute gun salute traditionally is fired starting at noon at all military installations with the necessary personnel and material. Guns will be fired at one-minute intervals. Also on the day of burial, those installations will fire a 50-gun salute -- one round for each state -- at five- second intervals immediately following lowering of the flag.
