I took a dictionary before I posted... but the wrong one as it seems. A German "Foreign Words Dictionary".
The spelling depends on from which word you make it.
If it's Italian-based (the original is "detto" in that case), ditto makes sense.
If it's Latin-based or taken from a French variation, it's "dito" (that's the way it's written in France and Germany).
The Oxford Dictionary has it "ditto". So it's language based.
Funny thing. I thought it should be the same in all languages, but it is not.
However, "dito" looks more classic.

After all, I'm pro Europe, so I'll stay with dito.

PS: That google thing generally has two problems: 1) It shows names and words with other meaning as well. 2) It shows all the pages where people wrote it wrong.
Works with many words. Try many and manny for example. Only one is right concerning "a large amount of things", but both give lots of hits in google.
In our case, dito and ditto give about the same amount of hits, with some more for dito. That shows that both variants exist, as stated above.