Tropi,
my guess is that it's all in the definition : Davic runs up to 15 cars (NC1/2, non magnetic) with just a 10 A power source - but it's a beefy, stabilized DS unit (actually it's two 5 A units, one at +18 V and the other at -18 V )
With a wall wart, I suppose that the VA rating tells you just that the thing will not melt up to that value - but my bet is that voltage goes down the moment you start drawing current
Beppe