First it was 7 points solely if you round down McCain's number while rounding up Obama's (the actual percentages: Mccain 45.7% Obama 52.9%)... secondly, Dukakis was "blown out of the water" because he lost nearly every single state in the electoral college, not because of the percentage in the popular vote. Every state hated Dukakis, except a tiny region in the northeast, 3 states in the midwest and 2 states in the northwest. McCain outdid Dukakis by about double his votes in the electoral college.
The 3% swing is based on the 6% difference between Obama and McCain, if 3% voted McCain rather than Obama the result would have been a statistical tie.