Armor stands in 1.16 Pre-release 4 have inconsistent poses when placed or summoned using the /summon command, even when using the Pose NBT Tag to set the armor stand's pose. The Pose tag can sort of control the pose of the armor stand, but not reliably.

Replication:

Summon a few armor stands with a specific pose. (i.e. {Pose:{Body:[0f,0f,0f],LeftArm:[0f,0f,0f],RightArm:[0f,0f,0f],LeftLeg:[0f,0f,0f],RightLeg:[0f,0f,0f],Head:[0f,0f,0f]}}). Or you can just use regular old armor stands. If you look closely, the armor stands will have slightly different poses, and if you used the Pose tag, none of them will have the exact pose you specified. This is especially visible when the the armor stand has items on it. For example, the armor stands in the picture below were placed using the following command:

/give @p minecraft:armor_stand{EntityTag:{Pose:

{Body:[0f,0f,0f],LeftArm:[0f,0f,0f],RightArm:[0f,0f,0f],LeftLeg:[0f,0f,0f],RightLeg:[0f,0f,0f],Head:[0f,0f,0f]}

,ArmorItems:[

{id:"minecraft:golden_boots",Count:1b}

,

{id:"minecraft:golden_leggings",Count:1b}

,

{id:"minecraft:golden_chestplate",Count:1b}

,

{id:"minecraft:player_head",Count:1b}

]}} 1

Despite this, the /data command can set the pose of an armor stand reliably.