On average, countries with legalized prostitution report a greater incidence of human trafficking inflows.
If prostitution is legal rather than illegal, if sex workers and their customers can come forward and report it without fear of being arrested, and if the police can focus on sex trafficking exclusively and use tools like Backpage to help them locate it, do you think the visibility of sex trafficking is higher or lower? Without addressing this vital question, I wouldn't conclude that higher reports of sex trafficking means a greater incidence of sex trafficking.
Also, note how it says "human trafficking" and not sex trafficking. That is a common conflation. Why do you figure legalized prostitution would cause an increase in general human trafficking, of which sex trafficking is only a small percentage? Could this be a mere correlation and not causation?
Studies you refer to are called into question also because of the unstandardized and convoluted creation of such reports and what is counted in these reports. Different places define trafficking differently in making their reports and different places investigate deeper and have better estimates that others so how can you compare it?
http://journals.sagepub.com/doi/full/10.1177/0002716214521562
Some agencies have cast doubt on the very idea of producing macrolevel estimates. The U.S. Government Accountability Office ([GAO] 2006, 2, 10) and the ILO (2005a, 13, 14) have identified numerous problems with the way macrolevel figures are produced. And independent analysts criticize the use of different definitions of “victims” in constructing worldwide estimates; the practice of extrapolating from a few documented victims to the entire victim population; and “estimates” that lump smuggled laborers into the trafficking category regardless of their consent and conditions of labor (Gozdziak and Collett 2005; Jahic and Finckenauer 2005; Zhang 2009, 2012).
One conclusion is inescapable: The claim that human trafficking victimizes a massive number of people is unsubstantiated; it simply cannot be substantiated at the macrointernational level. The glaring evidentiary problems are so severe that even rough estimates of the worldwide magnitude of this hidden enterprise are destined to be fatally flawed. The same argument applies to national-level estimates.
So these higher reports are not necessarily reliable, are usually of human trafficking as a whole and not sex trafficking, and reflect the visible cases rather than the actual cases, many of which are more likely to be swept underground when prostitution is illegal.