Its all about value and being valued
I have purchased CSS Sculptor ($100 value) on the upgrade to CSS Suite you allow $50 for this previous purchase making the upgrade to CSS Suite $150 instead of $200. The 20% discounted CSS suite is $160 giving me just $10 for the $100 I spent not more than a few months ago. If you offered me the upgrade for $120 (20% off $150) I will still have spent $220 to get the CSS suite so I hope you can see my point.
I have rounded up from the 99cents to make it easier.
Best regards from a very wet England