Just got a Metal Halide ballast of 250w it 'says' 2.15A and for MH lamps.  I asssumed it can also run MV lamps. When I fire up a 250w Narva (made in Poland) lamp it was much dimmer than run on MV ballast. MH lamps have lower voltage than MV lamps but same current?

Its could be just a normal MV ballast, but yes it will run probe start halide lamps, as the current is the same.
A `Metal Halide` ballast doesn`t really exist from 70watt upwards, their either HPS ballasts, or MV ballasts, with MH printed on them.

Have you checked the lamp current? If it is lower than the rated 2.15A, check the capacitor (assume CWA).

It's a 230v series choke type and there are two current ratings..2.13A and 2.15A the print shows connection with super-imposed ignitor 4-4.5Kv.  

The difference between 2.15 and 2.13A is too small to cause any noticeable brightness difference (just 1%, the supply variations cause way more).
The question is, firstwhat current really flow there compare to the rating (but with serieschoke I do not see any realistic ballast fault causing the current to go down) and what was the current in your "reference" ballast (it could well have been damaged and feed the lamp by higher than rated current, so when the same lamp was operated only on the rated current, it appear dim)

I connected a 36w T8 flouro ballast in parallel with the 250w ballast. Measured the lamp voltage when fully warmed-up read 133v. I could not measure the current as my other DMM that can measure AC current is faulty. But 133v is the correct voltage but with additional 36w T8 flouro ballast in parallel? If the ballast were giving correct power to the lamp shouldn't the lamp voltage increase beyond 133v?

The 133V is what I would expect across a warmed up 250W MV lamp. That mean the lamp is (at least electrically) intact.

Once the mercury fully evaporate, mercury lamps have the arc voltage nearly independent on the current. That mean from about 60% till >200% of nominal current the arc voltage changes only by few percent.
So connectingthe extra choke won't change the voltage either.

So you really have to find some wayto measure the current, that is the only thingto judge the ballast.

I have informed the seller on the ballast problem and he suspects that the ballast may have been overwound by mistake. So he agreed to change a new one for me.

I don't think "overwinding" is even possible. Normally the winding fit there just snugly, even one extra layer won't fit there anymore.
Moreover I've never heard of a problem, when a machine will count a countable thing with any error (something else is, if e.g. the small nails are dosed by weight, there is some tolerance, but when the machine really count something, it just does not happen).

Other story would be the setting of the air gap: Thinner spacer orlarger force would make it smaller, so the inductance higher.

But in any case, the finished ballasts undergo an electrical test, where the impedance and insulation quality are measured (mainly withthe aim to detect shorts or lower breakdown spots, as these do happen quite frequently - when the coat on the wire is damaged). And I doubt such test will not spot the too high impedance as well...

But let see, when you get the new ballast...

I also doubt it's manufacturing flaw but more of branding which is a China brand btw I bought it because it is the only one I can find that is 230v (240v rating runs the lamps dimmer). It may be that this brand of ballast which also makes ignitor and lamps did it on purpose so if the user use other brand of MH lamps it's not as good as using their own brand.

I don't think so. I'm still convinced your "reference" ballast actually overdrive your lamp (due to an internal failure), so you become used to the higher brightness, so then the normal ballast appear to you as running the lamp too dim. That was the reason why I asked you to really measure the current, only that will be able to tell for sure, what is the reality...

"China made" will not make their own standard just to not be compatible. More in the opposite, they try (although with sometimes better and sometimes worse success) to be compatible with the standards, as that is the only thing allowing you to at least try their product...
The "vendor lock in" can afford only the top brands, who have sufficient market power and reputation to go on their own. That is for sure not the case for any "Cheepeese" maker anywhere around the globe...