I do it as follows: 1.059 is 59 points per gallon (ppg, a measure of sugar per gal). You had 7.5gal of this, 7.5 x 59 = 442 points total (the total amount of sugar in the pot). Then you boiled and got 5gal, so the new ppg is 442/5 = 88ppg (back to sugar per gal). This means the gravity post-boil should have been 1.088. You got 1.073. Boiling doesn't cause sugar to disappear sugar so some measurement is wrong. If your hydrometer was not reading right, I suppose that could cause the discrepancy.
You had 18.4lb or grain. 18.4 x 36 points per pound per gallon (ppppg) is around 662 points of sugar. I used 36ppppg just as an average for base malts and specialty grains. We'll use the low end of your efficiency since big beers generally give lower efficiencies. 662 x 0.65 = 430 points. Now divide by 7.5gal and you get 57ppg or 1.057. Thats close to what you saw. If we divide the 430 by 5gal, you'd have seen 86 ppg or 1.086, which is higher than your post-boil measurement. This suggests your postboil gravity measurement was wrong. Maybe you read it was 73 but it was really 83.
For your grain bill to match the post-gravity reading, you would have to have only gotten 55% efficiency. This is certainly possible with a big beer since more grain holds more water and that water has sugar in it. Plus you diluted with more water and didn't collect it (why? boil longer). I don't know if you brew a lot of beers with gravities like this, if you know your efficiency is 60% for a big beer then recheck your gravity.
Can you tell I'm bored here at work?