The number of stars relates in part to how well the car stands up to impact damage during EuroNCAP crash tests.If all cars were 5 star though, how would you know which is actually better?
And what do you want? A car that will avoid an accident or withstand an accident?
Details of each report can be found with pics via Google.
For example, EuroNCAP awarded the MG ZS threes stars partly after finding significant injuries could be caused to rear seat passengers.
This sort of thing is of critical importance to most given who normally occupies that space.
Most new cars these days are deliberately manufactured to achieve 5 stars as makers realise this is one deciding factor for most car buyers.
There seems no excuse for not achieving all 5 stars.