I have been working with the New York Passenger Lists, 1820-1957 at Ancestry.com.
I searched for Mueller, using the "exact" option and choosing "United States" Soundex. I got 27, 456 hits.
I searched for Miller, using the "exact" option and choosing "United States" Soundex. I got 131, 856 hits.
Why the difference? Mueller and Miller are Soundex equivalents. Or am I missing something? 
I guess what I was missing was that to get the same number of results is to leave the "exact" box unchecked with the last name when doing a search for Mueller and Miller.
In both of these cases, I get 314,500 matches (Mueller last name unexact), US Soundex Search. I also get 314,500 matches for Miller, last name unexact, US Soundex Search. I am guessing that 314,500 is an estimate rather than an exact number.
Ok, but.
I search for Bernard (first name, exact), Mueller (not exact), US Soundex--I get 318 results.
I search for Bernard (first name, exact), Miller (not exact), US Soundex--I get 316 results.
It seems to me that these two searches should return the same individuals. After all the names are Soundex equivalents of each other. I really like all the databases at Ancestry.com . But sometimes the searches...