It would be a reason to lower them, if it wasn’t evident that each of these vehicles performed similarly at their respective BR. Again, even the 20mm vehicles with ATGMs and the BTR-80 have such low stats.
Let’s say the U-SH 204 GK is the baseline for wheeled autocannon vehicles and the SUB-I-II is the baseline for tracked ones. Every improvement a vehicle gets over these baseline vehicles is worth a BR increase of varying degrees.
All 20mm armed vehicles have their own advantages over these two baseline vehicles, some have more, some have less.
The U-SH lacks APDS, is the slowest of the wheeled 20mm vehicles, no smokes and has only 2 crew members while being vulnerable to 7mm machine gus, so it has no survivability at all.
The SUB-I-II is slow, has an exposed gunner and it’s hull is vulnerable to 7mm machine guns.
All of the 20mm vehicles at higher BRs have better survivability, better mobility, extra features (thermals, ATGMs or stabilizers). It’s not just slight improvements either.
The R3 and AMX-10P have significantly worse survivability than the Luchs.
Compared to the R3 the Luchs gets a reverse gearbox, so while it is slower, it has more mobility overall and it gets APDS.
Compared to the AMX-10P the Luchs is a lot faster and thanks to its wheel layout, it isn’t much less maneuverable.
On top of that, it has a second driver, it gets thermals and scouting. (It also has better gun depression, but I don’t know if that ever really comes into play).
That’s not 0.3 higher than the R3 or same BR as the AMX-10P, that’s definitely enough to warrant the 7.3 BR.
The RCV (P), again, gets a stabilizer. That combined with better survivability than the R3 also is worth a lot more than 0.7 over the R3. Considering that the XM800T does better at 8.7, that should be a good sign that its BR is warranted as well.
Spoiler
If you want to get more kills with the RCV (P), bring it into higher BRs. Modern tanks have less side armour than the ones around 8.0.