This. Pre-release packs come straight from the box, while a tin of 4 packs may have gone through a lot of processing and each pack may have come from a separate box.
If you buy a box of cards, or get them from a pre-release, you will get 8 randomized packs from the box of 36 packs (usually, since 36/8 is not a whole number). An EX is obtained in about one in every, say, 12 packs.
Normally, if you buy 8 single pack blisters, considering each comes from a different box, (pretty probable) your chances to pull any EX is 1/12 per pack.
However, since EX's will average 3 in the box, and if you get 8 pre-release packs in the same box, then your chances are greater: 3/36 (1 in 12) for the first pack like last time, then 3/35, 3/34, 3/33 etc. for each consecutive pack. For every pack you open that doesn't have an EX, that is one less pack to calculate for in the pull rates. This is, of course, considering you have an average box, and not one where you get 2 secret rares in a box, or only 1 EX in a box (both of which I have gotten.)
So, your 8th pack from a pre-release has a 10.7% chance to contain an EX, rather than a 8.33% chance from a retail pack, a difference of more than 2%