Hi there and welcome.
It may be just because the engine is tight, and as the miles build-up, more power will be released.
But what I'd suggest is getting a back-to-back comparison with another van with the same engine.
If its the same, then its a characteristic "they all do that Sir", but if there is a difference, then ask the dealer why.
It'll put your mind at ease, one way or the other.
HTH
Paul