I don't think the mix they give is bad, but my advise is to design your encounters based on what makes narrative sense.
This, this, this.
Set up situations for the characters to engage with based on the narrative, and then let the encounters grow organically out of it. Encounters, like the rest of the adventure, should respond to the character's decisions.
For instance, recently my players caught a pair of kobolds infiltrating a town (an easy skill challenge), and learned of a sinister ritual that would be taking place in the kobolds' warren. I had already roughed out the total number of kobolds (~40), and was rolling to see what percentage were infiltrating the city, expanding abandoned mine shafts, or guarding the home warren each day.
The players decided to mount a frontal attack on a day when ~1/3 of the kobolds were at the warren, but they made sure to have the chief mine engineer and town guard lock down the city and collapse some of the abandoned tunnels, effectively cutting off any reinforcements. That decision made their foray to the warrens,
much easier. Unfortunately, they trigger an alarm trap after breaking into the warren, setting all the kobolds on alert, ready to ambush them (A much harder series of encounters, now). They played smart, though, and discovered a hidden side shaft that led to the kobold common rooms, which allowed them to flank the kobolds and surprise them, again gaining the upper hand.
If I had decided they were going to have n encounters of x difficulty, I would have had to ignore the consequences of the actions, basically railroading them through the encounters. Instead, they gained easier encounters overall by cutting off reinforcements from the warren, and then had a much harder single encounter after they alerted the warren, which the two of them won through luck and quick thinking.
If the characters had decided to hunt down kobolds in the town, or infiltrate through the tunnels instead, they would have had an entirely different set of encounters and difficulties.