Hmm... Mastering has certainly changed somewhat over the years. The most obvious being the transition from hardware to software, or rather the transition to 'include' software. When I was first mastering it was almost entirely hardware based and 50% of it was being pressed to vinyl. Now that was an entirely different ball game. It's still the case though that most of the top mastering houses use mainly hardware and there are some real pro mastering guys out there with years of experience and great skills.
In recent years there's been a lot of these 'online' mastering studios pop up. I sincerely doubt whether most of them simply aren't opportunistic self-proclaimed engineers that haven't got as much knowledge and experience as they would have you believe; nor the necessary hardware and studio environment to actually make that much of a difference.
The last 10% can sometimes be huge in terms of how a production translates to different playback scenarios and it DOES take experience and genuine knowledge to achieve that. There is an often overlooked aspect to professional mastering which is that it is another set of good 'fresh' ears to make final decisions. Production, particularly electronic production can be a long winded process that can render the artist numb to crucial areas and mastering can provide that final (ahem) fix. If one looks at how the high end commercial process goes (certainly for music with actual instruments) it nearly always follows the pattern of Recording Engineer - Mixing Engineer - Mastering Engineer. This is absolutely because collaboration with fresh ears, different environments and different skills bring more refinement to the table at each sitting. Truly, if the mastering process was a charade then it's without question that the corporates wouldn't be paying for its services because they value profit above all else.
A fiver though... If someone asked me to do a mastering job for that I'd not be able to tell em to go fuck 'emself for being bent over double laughing.