Size problem when exporting to PNG

I have this design I need to export as PNG with transparency and 4.200 x 4.800 pixels. If I select both the illustration and the background (an empty rectangle that works for the transparency), when I go to the dialog and enter 2.400 pixels for the width, the height value changes to 4.793, not to 4.800. But if I select only the empty background rectangle I get 4.200 x 4.800 pixels. What is wrong?

I've made a lot of designs with these specs without any issues, this is the first time I get this peculiar problem. TIA.