I don't know this for WGT, but a good physics implementation would base the initial speed of a putted ball on the strength of the hit, the mass of the clubhead, and the coefficient of restitution of the ball, just as they do for any other hit. It may be that the speeds are so low, at impact, that those factors are negligible, thus not used. I'd have to crank some numbers.
A good physics program for putting also takes into account the loft of the putter face, since the initial part of the roll is slightly in the air.