GTiff Output Mercator Corner Coordinates Error using 1.10.1

[itbeyond]

I am using Mapserver to generate GTiff files using the following:

PROJECTION
 "init=epsg:3857"
END

OUTPUTFORMAT
 NAME "GTiff"
 DRIVER GDAL/GTiff
 MIMETYPE "image/tiff"
 IMAGEMODE RGB
 FORMATOPTION "COMPRESS=LZW"
END

When I create a map it works as expected however upon looking at the gdalinfo I notice the coordinates for the map corners are correct in the DMS but wrong in the Mercator as per this (using gdal 1.10.1 and Mapserver 6.4.0):

Driver: GTiff/GeoTIFF
Files: mapserv.tiff
Size is 1000, 1000
Coordinate System is:
PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"],
    AUTHORITY["EPSG","3857"]]
Origin = (12889228.989798404000000,-3724386.063095235300000)
Pixel Size = (25.399986284008403,-25.399986284008403)
Metadata:
  AREA_OR_POINT=Area
  TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch)
  TIFFTAG_XRESOLUTION=200
  TIFFTAG_YRESOLUTION=200
Image Structure Metadata:
  COMPRESSION=LZW
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (12889228.990,-3724386.063) (115d47' 9.29"E, 31d42'10.76"S)
Lower Left  (12889228.990,-3749786.049) (115d47' 9.29"E, 31d53'48.88"S)
Upper Right (12914628.976,-3724386.063) (116d 0'50.71"E, 31d42'10.76"S)
Lower Right (12914628.976,-3749786.049) (116d 0'50.71"E, 31d53'48.88"S)
Center      (12901928.983,-3737086.056) (115d54' 0.00"E, 31d48' 0.00"S)
Band 1 Block=1000x2 Type=Byte, ColorInterp=Red
Band 2 Block=1000x2 Type=Byte, ColorInterp=Green
Band 3 Block=1000x2 Type=Byte, ColorInterp=Blue

If you take coordinate Upper Left as an example: DMS states: 115d47' 9.29"E, 31d42'10.76"S Merc states: 12889228.990,-3724386.063

The actual coordinates of the map are right in DMS but the Merc does not match the DMS after a WGS84 - Merc conversion as below:

DMS 115d47' 9.29"E, 31d42'10.76"S to DD: 115.7859 -31.70299

gdaltransform -s_srs EPSG:4326 -t_srs EPSG:3857
115.7859 -31.70299
12889227.4290409 -3724386.28296906 0

DOES NOT EQUAL
12889228.990 -3724386.063

This is not the same/correct coordinate pair and all the Merc corner coordinates in the gdalinfo are incorrect.

I believe mapserver is doing it right because if I project the map to epsg:4283 the corner coordinates are totally right, so I suspect mapserver is outputing it right.

​http://www.itbeyond.com.au/images/mapserv.tiff for reference.