I've never seen a hatch roofed car either. I wonder if it was aftermarket/dealer installed item. 

I have read that the discrepancy in production numbers comes from the idea that Pontiac actually built 1,377 white cars with Grand Prix interiors with the 400 T/A V8 and other Can Am spec components but only 1,133 were actually converted. It was Wanger's company doing the conversions so he would know.  The theory goes that these Can Am spec white cars were just sold into the regular dealer network so there were 244 cars roaming around out there that were ghost Can Ams.