When you enchant anything, the bottom enchantment slot is nearly ALWAYS the biggest number, and the top enchantment slot is almost ALWAYS the lowest. Anyone else think this is weird? If you want to test it out, just get a regular enchanting table without any bookshelves, and see how many times you have to click until the top enchantment box is a level 5 enchantment. I couldn't get it to happen after around 300 attempts.
Maybe the lower enchantment slot gives you a worse enchantability rating and the higher one gives you a better one? I have no idea, but one thing is for sure, there is an odd distribution going on here!
Essentially, the bottom enchantment (in this case its 40) is almost always the highest one. I think its actually impossible to have the top enchantment be a level 50 enchantment.
Could it be nothing? Absolutely! But why would Mojang go through the trouble of giving each enchanting slot a unique distribution if it would serve no purpose? It doesn't make sense.
What surprises me is that its not ALWAYS like this. The simplest thing for them to do would be to generate the 3 random enchantments and then organize them lowest to highest. But they have not done this, because sometimes a lower enchantment goes into the middle, as you pointed out in your picture.
The enchantment level is dependent upon the number of nearby bookshelves (capped at 30) and a "slot factor" of 0.50 for the topmost enchantment slot, 0.66 for the middle slot, and 1.00 for the bottom slot.
So yeah the bottom slot does have higher numbered enchanments, and the top slot does have the lowest.