Lockport is a city in Niagara County, New York, United States. The population was 21,165 at the 2010 census. It is so named from a set of Erie Canal locks within the city. Lockport is the county seat of Niagara County and is surrounded by the town of Lockport. It is part of the Buffalo–Niagara Falls Metropolitan Statistical Area.
Since 2010, the population of Lockport has remained stagnant. 
Overall, it is common knowledge that the world population continues to increase, but this is too small and specific of a location for that logic to correctly assume that Lockport's population also increased. It is reasonable for a small town to have a stagnated population just based on the enormity of possible life factors. Of course, it's reasonable to assume it went up or down as well by the same logic, which is why it could go either way. I think the system got it wrong because of the word that was used (stagnant), or maybe because it applied blanket common logic to the statement (i.e., "the world population keeps increasing, therefore the population is increasing everywhere", which is simply not going to be true in every single case based on how many cities/towns exist all over the world). 