I had mind to make a similar post, but decided against it. I still appreciate yours nonetheless because my main deterrant was in writing out how it's NOT 25%...also it didn't happen last year.
That being said...I'm not so sure your picture of the combinations is accurate either. It doesn't feel right, but I have no real argument against your description.
I went back to 2010-2011 in the Western Conference to find the last time it happened, but it was a lot more likely in the previous alignment with the 3 division winners and then everybody else. You just had to get #3 in the right spot.
In the current setup you would think the 1 v 8 matchup is always correct, but it's not because the 2 wild cards could have better points numbers than the other division's 1-3. See: Central Division. So it's pretty convoluted how it would need to come together.
It's a decent puzzle. I'm not sure the map you laid out tells the full story.
I think this year is the cleanest possible way to make it work. I think the most convoluted way for it to be fulfilled is below:
#1 from ATL (1) plays #2 WC
#2 from ATL (2) plays #7 from ATL (3) div
#3 from MET (1) plays # 6 who is #1 WC and must be from MET
#4 from MET (2) plays #5 from MET (3)
I also want to add that it's more than just playing the proper seed, they're also in proper order so that 2/7 plays 3/6 in Round 2 and 1/8 plays 4/5.