The left column of color wells in your illustration are applied to the stroke color for the area. The right column of color wells in the illustration are applied to fill color for the area. Both columns are not applied to objects in an undulating sequence as indicated by the sketched red path/arrow.
Try switching from interpolate to closest higher value to see the effect without any visual intricacy from interpolation.
If you look at areas that are colored by the 36.4 interval you'll see the fill color is a lighter pink and the stroke color for the area outline is a very dark, almost black red.
If you simultaneously add both stroke color and fill color to the intervals, then choosing a palette will automatically make the stroke color a darker shade of the palette. If you want to use completely independent palettes for stroke and fill color, set the thematic format separately.
You can do a shortcut for the above by first ctrl-clicking both wells so they both use the same thematic intervals, arrange those intervals as you like, assign the pallet you want for fill color, and then ctrl-click the fill color button off so that only the stroke color column remains. You can then assign the palette to that again and it won't be automatically darkened. That saves the effort of duplicating the intervals, copying/pasting sets of intervals in the mfd_meta table, or other shortcut methods.