I get this too. I have a friend named Dusty (reasonably rare), and when I try to type his name out, after typing "D" my iPhone often picks "i" even though I tapped "u". If I go ahead and finish it with "Disty" it _then_ gives me the option of choosing "Dusty".

Apparently, names that begin with Di outnumber names that begin with Du four-to-one.