It's probably because the OEM kernel is not included on the Live CD/USB image, so it is not available to use when you boot. Then, during install Ubuntu downloads the extra packages based on the hardware and options you chose. One of those is probably the OEM kernel packages, which has some extra driver modules to support newer Dell hardware than stock 20.04.
Now, the Dell Linux Recover ISO does include the OEM kernel. But I think that's only available for service tags that shipped with Ubuntu originally.