Stuck implementing k means for big and small dogs - dodgy results
The centroid is likely correct, you have a display error

The line

plt.scatter(centroid2[0][0], centroid1[0][1], color = "green")


should be

plt.scatter(centroid2[0][0], centroid2[0][1], color = "green")


That's just one of those things that happens when you implement an algorithm from scratch in order to learn it . . . I bet you've spent hours looking at the top part of your script :-)