Perhaps it's more based on determining what is a single attraction and basing it on that. Times Square is a very specific location in New York City, so to count it as "one" place is acceptable. People visit it to see the location itself, unlike the Vegas Strip. The Las Vegas Strip isn't in and of itself a destination, but rather the location of destinations people are visiting. But also, when people go to Vegas they say just that: "I'm going to Vegas." Since pretty much all there is to Vegas for tourists is the Strip, they chose that as the singular destination.
So it makes me think that since there is no way to really verify whether or not the Disney World parks' attendance accounts for the same guests visiting multiple parks in one visit they chose to use the numbers that were available to them per park as a more realistic scenario . If 1 guest visits four parks in four days, then that guest is counted four times in attendance totals for the resort as a whole...which inflates the number of guests visiting the resort (that is if you are combining the yearly attendance to all the parks to arrive at the figure).
If you look at it more logically, the odds are that everyone who visits the resort as a whole most likely visits the Magic Kingdom. While I am aware that some people do skip the Magic Kingdom as well as don't visit each park, those numbers are probably negligible overall. Depending on what numbers they are using, it's possible that they include park hoppers, double counting guests. What I mean is that if they base attendance on its peak numbers each day, that could occur (and probably does) at different hours. DAK could peak at 11:00am...and a person could leave that park at 12:00pm and get to MK, which peaks at 1:30pm that day. So if they use those "hard" numbers then guests are being double counted. There are probably people at Disney who sort that out...too bad none of them are on this board.
As to why they count Universal as one, who knows. That is a legitimate question. I didn't look at the numbers, but do they separate the attendance numbers for each park or is it simply listed as Universal Orlando? Theme parks don't release numbers, so these are all estimates based on insider information.
