Hoho I think I've solved the problem! Or rather, there wasn't any problem in the first place!
We all know the cruise control stalk comes from mercedes. Here's a page of a mercedes owner's manual regarding their cruise control:
IMPORTANT POINTS TO NOTICE:
The manual shows that the partial indent changes the speed by 1mph (1km/h) BUT IN REALITY, 1km/h is equal to 0.621371 mph.
Secondly, it claims to change the speed by 5mph (10km/h) for each full deflection BUT IN REALITY, 10km/h is equal to 6.21371 mph.
If you haven't already figured out the "problem", I just did an experiment that will prove that the "problem" doesn't exist.
Over here in my local area, we use the metric system just like every other country I've been to except for the USA.
I took my Dad's mercedes out and I'm used to the cruise control on this ML-class Mercedes.
Every partial push = 1km/h, and each full push = 10km/h change.
There has never been any intermittent behavious with the numbers on this car but here's the interesting thing - I can select METRIC or IMPERIAL units from the car settings.
I went ahead and changed the setting to IMPERIAL and although my analog display still showed km/h, the digital speed display started displaying in imperial units.
I took the car up on to the expressway (highway in american) and engaged, disengaged, and adjusted the cruise control several times.
HERE COMES THE REVELATION:
After having driven the car for more than 2 years with a consistent cruise control that always gave me 1km/h or 10km/h changes, now full pushes of the stalk sometimes gives me 5mph and sometimes gives me 6 miles per hour in change of speed according to the digital speedometer.
If you haven't already figured it out - it seems that daimler-designed cruise control is based on the metric system and NOT the imperial system.
The screenshot above of the MB owners manual is the ONLY version that shows "5 mph (10km/h)". If you pull up the same owners manual from Mercedes UK, or any other metric-based country, it simply says "10km/h". There is no mph in brackets or anything.
In simple english, it means that the car will APPROXIMATELY change the speed by 5mph (but in reality it's 10km/h)
And depending on the current speed and changed speed in km/h, the display mph has to be rounded up or rounded down to a whole number. Example if the result is 62.3 mph you will see 62. If the result is 62.6 mph you will see 63. So sometimes you end up seeing 1 more or less than you think you ought to see.
Lastly, I don't know how the metric version of the Tesla Model S works (dont have one yet) but if that is not doing 1 / 10 accurately, it may look like Tesla engineers designed the car around the imperial system, converted daimler's metric data to imperial to work with the rest of the imperial sensors and stuff and when someone selects metric units, the car then takes the imperial data and converts it to metric again (double conversion). If any rounding is done in the middle, there might be a discrepancy.
Of course, this is all speculation based on daimler/mercedes and may have NOTHING to do with Tesla... It could just be a daimler hardware part with 100% tesla software and codes so I could be wrong...