It sounds like a bug or a really stupid decision by game devs.
And it's very easy to fix with the obvious fix.
Veterancy bonuses shouldn't be absolute ffs but relative. So vet 1 gives you 20%(or whatever is deemed right) increase to the basic values, vet 2- 40% and so on.
Then when you have 5% for a unit without stabilizer + 20% of that 5%= 6%. With vet 4 it will be 9%.
On the other hand if the unit has stabilizer and his basic accuracy is 15%. With vet 1 it will be 18%. And with vet 4= 27%.
So the veterancy will still be handy for any unit but the balance between vetted cheap and elite units will be the same as with unvetted ones.
Really can't see any reason why it wasn't done in the first place.
This is probably the simplest way of solving the problem of unequal power of veterancy, make it a percenatage and all units will benefit equally to veterancy. The accuarcy bonus should be 25% per level to be on par with the cost increase, maybe 30-35% for it to be an attractive choice (if few high quality units kan kill exactly the same amount of things as many low quality units, it's better to get the low quality units because they're harder to kill).
It would be necessary to revisit the accuarcy/cost of a few units that really require vet to be functional though(chapparall, I'm looking at you).