Are the torque numbers different? It IS a diesel.
When you put larger turbo, it doesn't matter if it's a diesel or petrol, you will get more torque at higher rpms and thus more HP.
In fact, torque numbers are even lower for stage3 compared to stage2 (147 for stage 2 and 121 for stage 3). However, the graph shape for stage 3 is correct (torque 'extends' further down the rpm range), but the total value of the torque is too low. And thus the final HP output for stage 3 is 1HP less than stage 2.
I guess the PD's mistake was the wrong peak torque value for stage 3. Torque curve is correct, just the absolute value is too low.
Update:
If we look at all three turbo stages, we get the following results from the game:
Stage1: torque - 132kgfm @ 2000 rpm
Stage2: torque - 147kgfm @ 3000 rpm
Stage3: torque -
121kgfm @ 4000 rpm
All these numbers correspond to the theory very nicely (except for the peak torque number for stage 3). Indeed, if you put bigger turbo, you'll get higher maximum torque (and power) at higher rpms.
If you look at difference between stage 1 and stage 2 - the bigger turbo from stage 2 adds extra peak 15kgfm of torque
at extra 1k rpm compared to stage 1. The same should be upgrading from stage 2 to stage 3. And we do get higher rpms there, but we don't get higher peak torque. We don't get even the same peak torque. And that's were the bug is.
OK, so much for my rumblings today. Let's go racing