Note that the style rules are displayed in a tree which
represents a progressive refinement of styling. As well as the meaning
within each style rule, there is also meaning in the tree structure.
In fact, the style rules are displayed in ascending order of specificity
(least specific at the top of the tree).
The population<100000 rule
is more specific than the population exists and not null rule. Therefore the population<100000 rule has priority. The settings in the population<100000 rule override settings in the node rule for all nodes that have
population less than 100000.
You can move rules up and down the tree to change their
priority, but you cannot place a more specific rule above a less specific
one. The commands to move rules are on the Rule Menu which is available
when you right-click a rule.