A toilet bowl is sort of like a teakettle...it should hold water until the level exceeds the outlet of the spout (in a toilet, it's called the weir). If you pour the water fast, it DOES act like a flush, and the level will be lower. When you flush, the fill valve directs water not only to the tank, but also into the bowl to refill it. If you pour the water slowly, it won't active the siphon, and it will slowly just go over the weir and only fill to the 'normal' full point. IOW, what you saw is probably normal, unless it slowly drains over time by itself.