As an example of LEAVER being read instead of SEAVER, the 1900 census soundex for MA has about 38 cards for LEAVER or LEVER.Of these, I was able to identify 7 cards as SEAVER persons based on my research in MA VRs and other census data.
There were about 14 LEAVER/LEVER cards for people from England and Germany, so the ratio is 7 out of 24, or almost 30%.
I've noticed a similar ratio in the 1850 census data also.
These errors have occurred because the script L and script S were very similar for many census takers.The people who indexed the data had to make educated guesses and sometimes guessed wrong.
If anyone is missing a SEAVER family in the 1900 census for MA, let me know and I'll check the LEAVER data I have.