....of how voters going into the various caucus sites were likely to vote, state-wide.
Two things they do not take into account are:
Breakdown of voters in each district
Effect of the caucus itself - the short speeches by each candidate's representative.
Sanders had very strong support in a few districts where there were many college students and much weaker support in many of the other districts. Clinton, on the other hand had more uniform support throughout the state.
So, Sanders could have won his few strong districts by 90-10 (an exaggeration) but lost in most of his weak districts by 52-48 (another exaggeration). But all the "excess" support in those few districts was essentially "wasted" because he'd already won those districts.
This is a bizarre analogy (please don't laugh at me!), but let's look at the 1960 World Series between the NY Yankees and Pittsburgh Pirates. Think of each game as an Iowa district.
Here are the scores:
New York Yankees 4, Pittsburgh Pirates 6
New York Yankees 16, Pittsburgh Pirates 3
Pittsburgh Pirates 0, New York Yankees 10
Pittsburgh Pirates 3, New York Yankees 2
Pittsburgh Pirates 5, New York Yankees 2
New York Yankees 12, Pittsburgh Pirates 0
New York Yankees 9, Pittsburgh Pirates 10
The Yankees scored more than twice the runs than the Pirates, yet they lost the World Series because they won three games by huge scores (+13, +10, +12!) while the Pirates won four games by smaller margins (+2, +1, +3, +1)
The result is that even though outscored overall by 30 runs the Pirates won the World Series. That, to an extreme, is how things went in Iowa on Monday. Sanders won some districts by huge margins, but Clinton won more districts by smaller margins.