I tried to add people in a certain area to a certain team, but when I went and tested it, the command didn't seem to work.

When I tested it again, but this time with a /say command, I discovered the commandblock uses the wrong coordinates (I couldn't find out which. I walked for 5 minutes with speed 25 and I couldn't find a place where it wouldn't find me anymore)

Steps to reproduce:
1: generate a superflat world
2: have a commandblock continuously run this command (It's the one I used): /say Hi @a[x=1020,y=4,z=-522,dx=1028,dy=2,dz=-518]
3: go into the area where the commandblock should find you according to the command (when I tried this it found me, but it didn't stop when I got out of that area)

4 (optional): use /setblock 1020 4 -522 glowstone and /setblock 1028 2 -528 to see the area in which the commandblock was SUPPOSED to find you