small difference in projection between tilemill and postgis

I would like to create a world map with interaction on continents created directly from a postgis database.

For the moment, I have a POSTgis database with the data. I created a background with TileMill. I get data from Postgis in SVG and put them on the map using Raphael.

All seem well except that I have a small difference between both map :s I use default projection of TileMill (and I think we can't change it :)). And here is the POSTGIS command I'm executing to get the SVG:

SELECT st_asSVG( 
      st_scale( 
          st_translate( 
              St_Intersection( 
                  ST_Transform(ST_SetSRID(ST_GeomFromText('POLYGON((-151.875 80.6470,175.7812 80.6470,175.7812 -63.2336,-151.875 -63.2336,-151.875 80.6470))'), 4326), 3857), 
              (SELECT st_union(geom) FROM geo.continentboundaries)) 

          , 20037508.72, -20037508.72) 
      , 0.0000199043047125,0.0000199043047125)
    , 0, 0)

Here is the result: http://i.stack.imgur.com/7lS0F.png

Do you know where I have a mistake?

Edit: some precisions: I'm using the same data for Tilemill and the svg. I added in the tilemill project the example of tilemill there is no problem of projection. In the layer in tilemill I'm using 900913. st_srid give me 3857 in Postgis which seems ok? I'm a beginner in SIG