I just talked to a rewards person and this was explained to me as such:
Platinum is earned by getting 75 nights.
A formula is applied to all platinum's and the top 3% of those are Premier Platinum. They get the recognition but no additional benefits.
Lifetime Platinum had an old formula and now a new one that says one doean't have to earn platinum every year to enjoy the benefits.
I have been Lifetime platinum for a lot of years and now have qualified again by the new formula but my account has and still shows platinum because I have continued to earn the level. The reason for this is hotels look at platinum as higher than lifetime platinum since they know that you earned it and are a more frequent stayer. If I fall below the 75 nights needed to renew platinum than my account will show lifetime platinum.
I hope that I have made this clear!