I'm going to take a guess here, but I can think of a few reasons why the timber might be left proud where it meets the receiver.
1. It's in the way the stock fits into the metal with a sort of ball and socket type arrangement. If the wood were fitted tight, shrinkage over time would leave sharp exposed metal edges.
2. If the wood were fitted tight, given the dimensions of the receiver the wrist would feel too small for a good number of shooters.
3. If it were fitted close, the cost of the rifle would be much higher than it is as that takes extra time and time is money.
Pretty much boils down to speed of production as mentioned above.
Diego, should you choose to reshape the wrist or pistol grip, take notice of the buttstock hole and its relationship to the surface in that area. If you drop the comb detail into the wrist area for asthetics you risk thinning too much and creating a weak spot that may split over time. Be sure to leave enough 'meat' for the sake of structural integrity. "Pallet wood" stock blanks aren't exactly selected with fine detail in mind.