text search synonym dictionary anomaly with numbers

I am working with street address data in which 'first st' has been
entered as '1 st' and so on. So I have created a text search
dictionary with entries:
     first  1
     1st  1
And initially it seems to be working properly:

SELECT ts_lexize('rwg_synonym','first');
 ts_lexize
-----------
 {1}


SELECT ts_lexize('rwg_synonym','1st');
 ts_lexize
-----------
 {1}

But my queries on '1st' are not returning the expected results:

 SELECT count(*) FROM parcel_attrib WHERE txtsrch @@ to_tsquery('1');
 count
-------
   403  <- this is what I want

SELECT count(*) FROM parcel_attrib WHERE txtsrch @@ to_tsquery('first');
 count
-------
   403  <- this is also good

 SELECT count(*) FROM parcel_attrib WHERE txtsrch @@ to_tsquery('1st');
 count
-------
     4  <- this is not good. There are 4 records that do have '1st',
but why am I not getting 403 records?

Thanks for reading,
Rich

--
Richard Greenwood
richard.greenwood@gmail.com
www.greenwoodmap.com