The front of oven is wider than width of cutout . In this case cutout width is same as the board above oven in pic.
I think you might have a 24" oven,27" is an odd size that I've never incountered. At any rate to see if the new oven can go in place of old,slide old out a few inches. Measure width between cabinet sides and compare with new oven measurements. If sides are far enough apart you can trim face frames to larger width.

27"w is very common and GE (manufacturer of this one) made several of them in the '90s when this appears to date from. Although the MW looks like a 24 incher in the pic but hard to tell without the bottom oven in view. If there's 28-3/4" between the walls of the inside of the cabinet above the microwave, a 30"w oven/MW combo should fit.