Prime brother/blaster brother discrepancy

Hello, I was building a small Firefight team and noticed the following discrepancy:

You can upgrade a Prime brother with a plasma rifle for free. The Prime brother then ends up more expensive that a Blaster Brother and less well armed. How come? What have I missed?

Upgraded Prime Brother: 6" 3+ 2+ Plasma Rifle (24”, A1, AP(2)) CCW (A2) Fearless = 50pts
Blaster Brother 6" 3+ 2+ Heavy Plasma Rifle (30”, A1, AP(2)), CCW (A2) Fearless = 45pts

These sort of discrepancies happen due to the way we round point costs for GFF, and unfortunately they pop up across multiple armies with specific upgrades.