Background is not pure white

I recently had a poster printed on an offset press that contained two images of music created in Musescore. To my surprise, the music printed with a background that was not white; it was a pale grey.

I traced the problem back to Musescore itself. The background color is not white (255 RGB measured on a digital color meter) but pale grey (249 RGB). This difference is barely visible on a monitor, but it becomes apparent on matter printed on a high quality press.

I went into preferences to change the background color. On the Apple Color Picker the white swatch reads as 255 RGB, but when selected, Musescore defaults to 249 RGB. Not only is the sheet background 249, but all pallets that show white are also 249. None of my other applications behaves in this way.

I'm using version 2.3.2 and cannot upgrade because I'm using an older computer that contains valuable legacy software. The white problem is easily correctable in Photoshop, but I'm wondering why it exists in the first place.

Thanks for any information.