The law seems to be made so complicated that a judge can always find an excuse for a lenient sentence. It also always seems to put the offender first, the offender says albeit this being the 28th time in 16 months(random example) this time he's really gonna learn from it and the judge dishes out an ''discount for good intentions a guess?
Actually, it's a good deal simpler than you think. The Judge looks at the sentencing guidelines for the offence, which are fairly clear.
As mentioned above, the maximum sentence for careless driving is 5 years, so he can't have been done for that. So lets look at causing death by dangerous driving.
The Judge must decide whether the offence is Level 1, 2 or 3. 3 = significant risk of danger. 2 = substantial risk of danger, 1 = most serious (flagrant disregard for rules of the road, apparent disregard for the great danger being caused to others). Each of these has a starting point and a range. So 3 = 3 years, 2 = 5 years, 1 = 8 years as a starting point.
The Judge then considers aggravating factors and mitigating factors. In this case the aggravating factors are previous convictions and consumption of drugs before driving. Mitigating factors are things like "i was driving because it was an emergency" or "i didn't know that I still had drugs in my system, or someone spiked my drink".
If the defendant pleads guilty from the get go, then they get a reduction of one third of the sentence.
Ancillary orders are considered and the sentence has to be proportionate to the offending behaviour (totality guideline).
So in this case, the Judge may have started at level 1 (8 years) increased to 10 years for the aggravating factors but then applied a 1/3 discount for an early guilty plea - which would result in a 7 year sentence. Or started at level 2 pushed up to the maximum and denied the reduction for a guilty plea as the defendant was already on bail for another custodial offence.