hob worktop dimensions

Hello peeps, I’m about to embark on cutting a hole in a worktop for a ceramic hob. According to hob manual, external dimensions are 600 x 510 (mm) and the hole dimensions are 560+4+1 and 490+4+1, respectively. I find this a tad confusing and I’m not sure why it doesn’t just say 565 x 495 for...