Yes, it can be deceiving. I've tried to run water up hill more than once in a field terrace channel before we got the proper equipment to actually measure grade.
What made it worse in your case is having the water low so long there was no close reference line to observe. Had the pond filled normally and got to full pool level, you might have noticed the low spot then with the water level up closer to it.
Too much water all at once never gave you the chance.
