Why does my compiled sass output differ from released ionic.css - resolved : remove gulp-minify-css

when compiling the ionic scss source the result differs from the released ionic.css - see examples below.
I am assuming the issue is a different sass compiler - I am using gulp-sass 2.1.0 installed via npm 2.7.4
what is the distributed ionic.css compiled with ?
I am missing some sass compiler configuration/option ?

this came to light when digging into why footer bar styling was messed up ( half height colour )
I found that background-image becomes background & background-size is dropped altogether:

ionic.css :

.bar.bar-stable.bar-footer {

background-image: linear-gradient(180deg, #b2b2b2, #b2b2b2 50%, transparent 50%);
}
.bar {
. . .
/* border-width: 1px will actually create 2 device pixels on retina */
/* this nifty trick sets an actual 1px border on hi-res displays */
background-size: 0;
}

compiled.css :

.bar.bar-stable.bar-footer {
   background: linear-gradient(180deg, #b2b2b2, #b2b2b2 50%, transparent 50%);
}

.bar {
. . .
/* border-width: 1px will actually create 2 device pixels on retina */
/* this nifty trick sets an actual 1px border on hi-res displays */
}