Going to offer two ways of understanding this halacha which does sure need explanation, and I apologize in advance for the length.
One is along the lines of what was said before but I once heard the following mashal which brings it out. A friend of mine told me he was once getting a ride with someone who was always very careful to drive 2 miles under the speed limit (we all know someone like that....). As they're driving up the GSP and the driver is of course going 63, all of as sudden they notice a cop parked on the side of the road running a speed trap. Instinctively the driver put his foot on the brake to slow down.... Even if you are driving within the speed limit, when you see a cop you slow down. Thats how we feel during this time of mishpat, even if there are areas in which we're ok such as pas palter which we can rely on, we feel the seriousness of the din so we slow down.
Secondly, I personally understand it best with the following story, there was a chassid of the Rebbe Rashab who worked in a bank in Saint Petersburg during the week and came back to Lubavitch for Shabbos. During the week he was of course dressed to the nines, 3 piece suit, top hat and coat etc., but for Shabbos he dressed in the simple clothing of the other chassidim in Lubavitch. One week he thought to himself "who am I fooling, I dress a certain way all week, I will dress that way on Shabbos as well". After Shabbos the Rebbe called him in and said "Until now, I thought that you are who you are in Lubavitch and you're fooling them in Peterburg, now I see that you are who you are in Peterburg and you're fooling us here in Lubavitch". The moral of the story: Evry person puts on "different clothing" so to speak in different times in their lives, based on the environment, surroundings, what he's trying to accomplish etc. Who's to say that who we really are is not the person we are during the Aseres Yemei Teshuva with all the extra chumros and hiddurim, and the rest of the year we are fooling the Yetzer Hara by "dressing" differently?
Lchaim!