Key thing is this map, which shows that summer temperatures have increased significantly in some parts of the country, but not others.
It's worth nothing that these regions are continental and already generally hot during the summer.
"Global warming" is not an accurate name and has been latched onto by some on the right to dismiss whats going on. It is "Climate change". The overall average global temperature is certainly getting hotter but it's not universal to every point on the earth.
However there are lots of other major impacts beyond average temperature such as changes in the lengths of summer, wetter or dryer summers, wetter or dryer winters, warmer winters, more unstable weather such as hurricanes, storms etc and more.
So while the average temperature in parts of continental North America may seemingly be stable, it's extremely unlikely there haven't been other changes. Earth is a single connected climate and biosphere; lines drawn on maps are irrelevant. This is only "strange" if you take a simplistic view that temperatures should be going up everwhere.