Why would you have to pay a premium for the digital version? Is that the norm these days?
Always has been, physical copies are cheaper by virtue of the fact that retailers can negotiate pricing and they don't want to be lumped with massive amountss of valuable stock. A digital version has no value until sold.
That said, it's only recently that the gap seems to have closed somewhat with physical prices having gone up substantially on new releases. I guess it had to happen eventually.