Okay. Look at it like it like this.
Just because the odds are 1/5 doesn't mean that you're going to always get one. See, you'd always get your lvl X out of 5 packs if the boxes were divided like this.
(Y=Something of value N=Nothing)
YNNN
NNYN
NNNN
YNNN
NNYN
See? Every 5 boosters has one of the lvl Xs. If every buys 5 packs from this box, everyone gets a lvl X. But this doesn't always happen. The box could be divided like this:
NNNN
NNNN
YYNN
NNNN
YNNY
Fresh box, they pick you 5 from the top. As you can see, the top row + 1 from the next row leaves you with no Lvl X.
However, they might then go get 5 boosters for the next person. That guy just got 2 Lvl Xs! He had the same chance as you, but he was luckier.
Or of course, the box might only have 3 in it, greatly decreasing the odds.
Once again, chance is avery weird thing!