According to your logic then the cars with a 0-8 gauge have 2 bar more than one with a 0-6

No Lee, I would not say that.....firstly, why they have different gauges, I do not know....possibly the earlier Alps had the 0 - 3 - 6 gauges, but thats just a guess, you will know better than me on that point.
With regard to them reading 2 bar more, the electrical signal will be at a value decided upon by the sensor, the value of which will be displayed on the gauge at a point irrespective of the range of the meter. ie:- if it senses say 4 bar on a 0 - 3 - 6 gauge, surely it will read one third distance between the 3 & the 6...assuming everything is working correctly

1990 GTA Atmo, 2003 Jaguar X type 2.5SE Auto, 2018 Kia Picanto GT-Line-S 1.25