x,y,z,dx,dy,dz don't always work correctly with c=1

Executing this commands results in two outputs "test":
{{/summon armor_stand -6 54 34

{NoGravity:1}

}}
{{/summon armor_stand -5 54 35

{NoGravity:1}

}}
/execute @e[type=armor_stand,x:-7,y:51,z:33,dx:10,dy:3,dz:10,c=1] ~ ~ ~ say test
Because the target selector contains c=1, it should just give out one output, but it gives two. Interestingly other coordinates correctly just give one result, for example this:
/summon armor_stand 1 1 1 {NoGravity:1}
/summon armor_stand 2 2 2 {NoGravity:1}
/execute @e\type=armor_stand,x=0,y=0,z=0,dx=5,dy=5,dz=5,c=1] ~ ~ ~ say test
...gives just one output "test". I also tried both coordinates (x and z) going over the +/- mark, just one going over, both being completely in the negative area and much more, but I could only reproduce it with those coordinates. But with them it's consistent.