[Inkscape-user] Import/Export of bitmaps

Could someone offer guidance as to why a bitmap (PNG) measuring 600 x 350 px becomes 640 x 373 px after a round trip through Inkscape 0.45?

I'm running on Windows XP SP2 and my objective is to add some drawing elements to a bitmap image (in their own layer(s) and within the dimensions of the bitmap). The original image was created at 96dpi, however it appears to "grow" on import to Inkscape. Inkscape preferences are also set to export at 96dpi, resulting in the over-sized new bitmap.

Thanks for any pointers.

Peter HB