Simpl answer, the engine in your car isn't the orginal one. Looks like the car has had another engine fitted at some time in its life and from looking at the tag it wasn't a GM VX-PRO engine.
From what i can remember, and this is going back a few years now so it might have changed, but when someone/company rebuilds and engine and sell it on there is/was no legal requirment to issue it with an engine number.
A few years back a couple of big engine re-builder in the smoke were raided and had all the stock and tools seized because there wasn't any number on the engines they sold. It all went to court and it was ruled that they are not a legal requirment. I don't know if this is still the case though.
The GM engine numbers are in two sections, eg,
X30XE
The X = euro 2 regs
30 = engine size in CC
X = comprssion ratio 10 to 11.5:1
E= multi-point injection
The second part it the engine serial number and thats what we use it ID any changes made during manufracture.
With a non Genuine re-built engine the builder will grined off the OE number and stamp his owen build number on it for his records.
If your buying a car, they always say check the numbers against the log book, fair point regarding the Chassis number but all the engine number will tell you is if thats the orginal engine. If the numbers are different then you start asking questions
By giving a VX dealer the reg or chassis number he will only come up with the orginal number, its quite possible if its a pattern engine you could have a late car with an early spec engine or the other way round
Hope that sort of helps
Cheers
Andy