Found this somewhere:
Cars can undergo inspection at any time an FIA Technical Delegate and his scrutineering team deem necessary. Car selection is done at random, by computer, throughout the course of the meeting to ensure fairness in vehicle inspection.
Apart from what @Kryptos says, itâs about the wear on the plank they have to bolt under the floor. It needs to have a minimum thickness of 9mm left at the end.
As itâs to do with wear itâs also possible Russelâs setup had slightly more ride-height or he avoided the bumps and/or kerbs more with the lines he drove.
The other two cars checked were those of Norris and Verstappen. Both were compliant.
Checking MV, LH, C LeC, and LN (only?) doesnât sound very random.
Seeing as how many of the cars checked, failed, it would seem to indicate a wider problem and all of the cars should have then been checked.
It could be a track problem causing the wear on all cars, not a car set up failure as such. Unfair to then penalise just a few.
I had the same initial thought, that perhaps itâs the podium and one random they check?
But I havenât found anything to support that theory.
Dunno what the full algorithm would be. Perhaps if it selects one podium guy they do the rest for fairness. Who knows.
But both Ferrari and Merc accepted the penalty with Toto saying the findings are indeed correct.
The findings may be correct but unless all cars are checked then the why may become a factor.
There may be many reasons for not all cars checked. It may be resource, cost and time driven. The scrutineering process will no doubt be extensively reviewed and agreed to by every team participating. If there was any recourse in the agreed ruleset Mercedes would no doubt have had made a point of it yesterday.
So my prediction is that it is not going to become a factor outside this forum.
I think that the plank may be titanium 
But that makes a sport great in the us of a innit?
Yup, comment instantly debunked.
I think the plank may be wood.
A plank prank.
Both are true.
Skid blocks are titanium plates that are fixed on to a plank on the bottom of F1 cars. These planks were made mandatory by the FIA after the high-profile death of Ayrton Senna in 1994. The planks are designed to stop cars from being too low to the ground, thus improving the safety of the drivers.
But if the wear has gone further than the skid blocks into the plank they still donât need to measure anything. Takeaway, no wood was measured in the production of your F1 entertainment yesterday. Although no disclaimer can be made on whether or not wood was harmed during said production. 
Tells it like it is.
Nice clear explanation.
YUP - since titanium is so much harder to measure than wood.
Did not try to attract sarcasm with my comment. Please read the article posted by @Jamiewednesday, it literally stats that this is one of reasons. Perhaps you can assist the F1 to speed things up with your wood measuring skills.