Because I'm willing to bet that 9 out of 10 vehicles (GM or not) are not 100% accurate either. Nor are vehicles with different size tires that aren't corrected. To me, this is being nitpicky. Id also be willing to bet any dealer would run you off and not do anything about it, other than keep the vehicle for a little while to make you think they're working on it.
Theres also chances of a speed sensor, not gauge bad here, but 3% is probably within a margin of error that could be acceptable.
By all means, go get another cluster and see if it helps.
And if youre worried about a ticket - lets take worse case scenario - A 20mph school zone. So it says your doing 20 and youre really doing...... 20.6mph! Cop will NOT stop you for that. Period. His radar probably has a similar margin for error as well.
Im really not trying to be a dick here, but 3% is literally nothing, unless youre traveling over 100mph to make a difference. And without a tune, youre not going faster than 95-98mph anyways.