Yes, it's not rocket science.
The black tube on the fill valve also refills the bowl. If you turn off the water supply, then the bowl won't refill, and with low water in the bowl, it won't be a good flush.
The CST744S is a 900 gram bowl. I think the next step, would be to "use" the toilet for a week, and then report back. I don't see how flushing water is going to tell you anything.